ates with the axis vertical, the anterior pole being below, and the Monomyarian horizontal again. It must be remembered that the above considerations are taken in their most general sense, representing only the mean for each group, many of them perhaps erroneous. They are given rather for the purpose of indicating a further path of inquiry, which the writer considers fruitful and intends to follow, than as points in any way settled.

In ascertaining the mean position of the antero-posterior axis for the whole branch of Saccata, (that is, the average) we find that a line at an angle of 45° would represent its position in nature; the lower end being anterior. In the Radiates a line. through the mouth to the opposite region of the body would stand vertically. In Articulates the antero-posterior axis would be horizontal. Among the Vertebrates, Fishes would be horizontal, as in Articulates; Reptiles have the head slightly elevated; Birds and Mammals still more elevated; so that a mean line, for these classes might be drawn at an angle of 45°, the cephalic region being uppermost. Man stands vertical. Thus in a diagram we would have the following:

2.

Ma

Man.

Vertebrata.

Articulata.

Saccata.

Radiata.

In the preceding considerations I have endeavored to show the importance of the sac, as the principle and prominent feature in their plan of structure. All animals, reduced to their primary elements, are sacs in one sense of the word, though in one case a radiate sac, in another an articulate sac, etc. Yet nowhere does this character predominate so universally, nor is it expressed so simply as in the Mollusca; the leading idea as it were. It was shown also that, essentially, the heart is on the outer bend of the intestine, or between that and the sac wall, while the principal nerve mass was on the inner bend of the intestine. We would thus state their characters.

SACCATA.

(1.) Animals of varied forms, without a radiate structure and without articulations.

(2.) Stomach and viscera enclosed by a fleshy sac, which may be closed or open, at either one or both ends.

(3.) Principal nerve masses, consisting of ganglia, which are adjacent to, or surround the oesophagus.

(4.) Intestine bending inward, or having an outward flexure. (5.) Heart on the outer bend of intestine.

We must now consider the relations of the Saccata to the other branches of the Animal Kingdom. In the paper of Professor Dana, above referred to, he has used the terms alphatypic, betatypic, and gammatypic, as a numbering of the grade of types, whether of branches, classes or orders; also, below gammatypic, we have degradational; the Radiates are regarded. as degradational; and below this, hemiphytoid. He employs also, the terms used above, namely, Holozoic, for true animal forms, and Phytozoic, for plant-like forms.

Applying these terms to the classes or groups of Saccata, we have the following:

Prof. Dana has pointed out many interesting parallelisms be-. tween the groups of the different branches. Let us now look at the parallelisms between the groups above indicated, and the other branches. Cephalopods approach nearest the Vertebrates through their lowest class, the fishes, and already many interesting analogies have been pointed out between them.

Gasteropods may be likened to Articulates, through their lowest class, the Worms, through certain resemblances that many forms bear to the Leeches, Planarians, and Trematodes. Lamellibranchiates may be considered the essential embodiment of the branch to which they belong. Tunicates and Polyzoa may be compared to Radiates.

Or, in considering their freedom or fixedness in life, we have Cephalopods free, as in all Vertebrates; Gasteropods, a few fixed, as in Articulates; Lamellibranchiates, many fixed, as in Saccata, with relation to the other Branches. Tunicates, the greater portion fixed, though they do not compare so well with

the Radiates in this respect, but Brachiopods and Polyzoa fixed as in the lowest class of Radiates, the Polyps.

We would thus have

SERIES I. Represents a typical figure of each principal group in Mollusca-viz., P, Polyzoa; B, Brachiopoda; T, Tunicata; L, Lamellibranchiata; G, Gasteropoda; and C, Cephalopoda--(M, indicating Monomyaria of the second series). These figures are represented anterior end downward, the dorsal region being turned to the left. The tube within each cut, represents the intestine, the larger end of which is the mouth, and the smaller end the anus. The harp-shaped figure represents the heart, and the star represents the pedal ganglion.

SERIES II. Represents similar views, with less detail. The dorsal region in this series is uppermost, and the posterior end is turned to the left, as indicated by arrow A. The curved line indicates the intestine, the large end being the mouth.

SERIES III. Represents transverse sections of corresponding figures in Series II. SERIES IV. Represents the mean position in nature, of the antero-posterior axes of the figures represented above: A, Anterior pole; P, Posterior pole. The vertical rows of figures are identical.

ART. IV.-Petroleum in its Geological Relations; by Prof. E. B. ANDREWS, Marietta, Ohio.

IN the number of this Journal for July, 1861, I gave some facts bearing upon the geological relations of petroleum. My attention at that time was confined chiefly to those locations found in the Coal-measures of West Virginia and Southern Ohio. It is gratifying to know that the views presented in that paper have since been fully verified. As predicted, by far the larger part of the oil produced has been found along the axis of a wellmarked anticlinal, extending from the borders of southern Ohio, forty miles or more, into West Virginia, through Wood, Ritchie and Wirt counties. A smaller quantity has been found in the inclined rocks of Ohio; while scarcely a barrel has been obtained in horizontal rocks, although hundreds of thousands of dollars have been expended in the search. In this portion of our great Coal-measures the question has been solely one of subterranean fissures. The chemical conditions essential to the generation of oil have existed over a wide area; but the physical condition of fissures is found to exist in comparatively limited areas. Fissures serve two purposes, one to give space for the formation and expansion of the hydro-carbon vapors, and the other to furnish receptacles for the oil when condensed. These fissures must connect with the deeply seated sources of the oil. AM. JOUR. SCI.-SECOND SERIES, VOL. XLII, No. 124.-JULY, 1866.

If they have any surface outlets, by which the more volatile portion of the oil may escape as gas, the oil within them is thickened and lowered in specific gravity. As the escape varies greatly from different fissures, we find oil of every grade of gravity. I have known oil of 52° from a well one hundred feet deep, and also oil of 28° from the bottom of one eight hundred and fifteen feet deep. The Scott well on White Oak, Wood Co., West Virginia, struck, at two hundred and seventy-four feet, a fissure containing oil of 33°, and, at three hundred and ninety-one feet, another fissure yielding an abundant supply of oil of 274°. Hence, while, as a general rule, oil found near the surface is heavy, the fissures containing it being more likely to have surface outlets, yet sometimes the very deep fissures may have such outlets, and the contained oil be heavy.

The West Virginia oil field presents many points of great scientific interest. All the productive oil wells in this part of the State group themselves along the anticlinal line marked out in the article referred to, this line being the one of the greatest fissuring of the rocks. Toward its northern and southern extremities this line presents the form of a simple anticlinal with the rocks dipping on either side of the axis at angles varying from 5° to 25°. But in the middle part there is a double fracture, the lines of dislocation inclosing a somewhat ellipticalshaped area about ten miles long by one wide. These figures are only proximate estimates. A bird's-eye view would present an appearance somewhat like that given in fig. 1. The more important oil locations are indicated by the marks

1.

OHIO RIVER

A section of the dislocated strata is given in fig. 2.

2.

A, A represent the horizontal rocks. These belong to the highest strata of the Coal-measures. B, B represent the dislocated strata, inclining in opposite directions at angles varying

from 30° to 60°. Without having made any instrumental measurements I have estimated the thickness of these strata at about eight hundred feet. C gives the position of strata lying within what is popularly called "the break." These rocks belong to the lower Coal-measures and have been more or less flexed by lateral pressure. It is in these middle rocks that the most valuable wells of West Virginia are now being obtained. Wells bored in the rocks A, A, have been failures as also the wells bored in B, B. The rocks B, B, appear to have been lifted up bodily, and in such a way as not to have been much fissured. The advantages of the inner strata, marked C, as oil-producing rocks, are: first, they are bent and more or less fissured; second, they are many hundred feet lower in the series than the strata at A, A, and are consequently so much nearer the equivalents of the supposed sources of oil in the Devonian rocks of Western Pennsylvania and Canada; and third, this local disturbance of the rocks doubtless involves in its many fissures these underlying Devonian strata, and thus has given every opportunity for the generation of oil and its upward ascent. I think we may reasonably infer that the oil found along this line is of the same origin geologically as the oil obtained in the upper Devonian rocks of Venango Co., Pa. Thus far the oil obtained within. this double fracture has been found very near the inner edges of BB, as represented by the italics, a and b. These small letters indicate, respectively, the geological positions of the "White Oak" and "Mount's Farm" wells on the western, and the Handlan wells on the eastern side. The Volcanic Oil Co. and the West Va. Oil and Oil Land Co. own large areas of land within the "breaks." The "Mount's Farm" and other companies own smaller tracts.

I cannot but regard the term "volcanic" as infelicitous when applied to this region. Nothing is more sensitive to heat than. petroleum, and direct igneous action adequate to the work of uplifting and dislocating the strata to this extent would, I think, have driven off all the oil. The uplifted strata at C (fig. 2) contain seams of bituminous and cannel coal which possess the normal and average quantity of bitumen. There is, to my mind, a much better and more scientific explanation of this disturbance, one which assigns the cause to the lateral pressure produced by the subsidence of all the rocks of this region of country. This line of local uplift is found to be in the very heart or center of the synclinal part of our great coal field and at the summit of the coal formation. From forty to sixty miles both to the west and east the underlying strata, with their productive coal seams, begin to emerge. It would appear that at some time after the deposition of the last of the strata of our upper and barren Coal-measures there was a local subsidence which necessitated