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from the heart by two valves opposite to each other. Nor is this the only distinguishing character. A heart with a true conus arteriosus is always accompanied by a more or less developed spiral valve of the intestine (entirely absent in Teleosteans), and by non-decussating optic nerves. The fore and hind limbs of the Chondropterygians are also paddles supported by a cartilaginous framework; the tooth-bearing pterygo-palatine arch of the Dipnoi is homologically identical with the supper jaw” of a shark. And the anotomical evidence in favour of a union of Ganoids and Chondropterygians is rendered complete by the Holocephala (Chimæras), which differ in several important points from the other Chondropterygians, approaching the Ganoids by these very characters, and are, in fact, an intermediate form. They are sharks in external appearance and with regard to the structure of their organs of propagation. On the other hand, there is only one external gill opening on each side; the skeleton is notochordal, and the palatal apparatus coalesces with the skull as in Dipnoi, which is not the case in any of the sharks and rays; likewise the dentition approaches that of Ceratodus. Sir Ph. Egerton has drawn attention to another very important fact; viz. that the dorsal spine is articulated to the neural apophysis, and not merely implanted in the soft parts and immovable, as in sharks. Furthermore, all those modifications which show an approach of the ichthyic type to the Batrachians are found in Ganoids and Chondropterygians, none in Teleosteans; and, finally, the coexistence and development of Ganoids and Chondropterygians in geological epochs when no (or only very few) Teleosteans existed, is a circumstance which seems to confirm a conclusion arrived at from an anatomical point of view only ; namely, the conclusion that Ganoids and Chondropterygians should be united in one sub-class—Palæichthyes.
3. A third point of the deep interest is the great antiquity of the Dipnoous type. At the commencement of these notes we have seen that there is no evidence to show that the Barramunda is even generically distinct from those fishes, of which, unfortunately, the teeth only have been preserved. But some of the oldest fishes, known from the Devonian epoch, and designated by the names Ctenodus and Dipterus, prove to be Dipnoous fishes. They had the same dentition as Ceratodus, nostrils within the mouth, acutely lobed paddles, a notochordal skeleton, and, with exception of dermal scutes, a very similarly formed skull. Thus, then, we have the following facts before us: The Dipnoous type is represented in the Devonian and carboniferous epochs by several genera (Dipterus, Ctenodus, Chirodus, Conchodus, Phaneropleuron); it is then lost down to the Trias and Lias, where the scanty remains of a distinct genus (Ceratodus) testify to its presence; no further trace of it has been found until the present period, where it reappears in three genera (Ceratodus, Lepidosiren, Protopterus), one of which is identical with that of the Mesozoic era. Now, at present scarcely any zoologist will deny that there must have been a continuity of the Dipnoous type, and it is only a proof of the incompleteness of the palæontological record that we have to derive all our information regarding it from only three so very distant periods of its existence.
In conclusion I may add a synoptical table, from which the systematic views advocated above, and more especially the position of Ceratodus in the system, may be readily understood. After the separation of Amphiosus and the Lampreys as types of two distinct sub-classes (Leptocardii and Cyclostomata), the remaining host of fishes are referred to two other subclasses :
SUB-CLASS : Teleostei. Heart with a rigid bulbus aortæ; intestine without
spiral valve ; optic nerves decussating (living species, nearly 9,000). SUB-CLASS: Palaichthyes. Heart with a contractile conus arteriosus; intestine with a spiral valve ; optic nerves non-decussating. Order I.-- Plagiostomata,or Marine Palæichthyes (sharks and rays; living
species, nearly 300).
Sub-order 1.- Amioidei (one species).
Sub-fam.- Protopterina (Lepidosiren, Protopterus).
BY JAMES CARPENTER, F.R.A.S.
THERE are few scientific institutions whose objects are so 1 little understood, and whose labours are therefore so likely to be misjudged, as an astronomical observatory of the character of the national one at Greenwich. Even those who possess some knowledge of astronomy, who read its literature, and take a warm interest in its salient achievements, are frequently little or not at all conversant with those departments of the science that are perforce pursued in an essentially practical establishment, where the sun may be observed day by day without a moment's thought being given to his spots, the moon watched by night without a care for her physiography, and where the planets and stars are subjects of a system of close observation which, however, gives no heed to questions concerning their physical nature.
It happens, from a circumstance that will bye-and-bye appear, that the present is an opportune time for reviewing the history of the Observatory at Greenwich and its relation to current astronomical science; but it may be mentioned that the appearance of this article at this opportune time is merely accidental.
To start with a just idea of the very definite aims of the Observatory, we should clearly recognise the circumstances that led to its foundation. It was born of a necessity that arose from that extension of British navigation which was, at least partially, a consequence of the passing of the Navigation Act of Charles II. The necessity was a means of obtaining the longitude at sea. The latitude, we may remark, presented no difficulty whatever. A method for longitude had for more than a century existed in theory; for Apian in 1524, Gemma Frisius a few years later, and Kepler subsequently, had proposed the use of lunar distances in the very form that now universally obtains. The method may be thus described. The moon moves rapidly among the stars. Suppose that for a given instant of Greenwich
time her angular distance from a near star be known beforehand. Then if an observer at sea measure the distance minute by minute till he makes an observation which shows the distance * equal to the given one, he knows the Greenwich time for the moment of that observation; and this time compared with the local or ship's time of the observation gives, by mere difference, his longitude reckoned, in time, from Greenwich. If the distance of moon from stars be given at fixed intervals of Greenwich time, say hourly,t then it matters not when the navigator takes his distance, for he can always find, by interpolation, the instant of Greenwich time corresponding to the moment of his observation.
Now, to make this method practical two things are necessary. First, the positions of the fixed stars must be exactly known; and second, the moon's place hour by hour must be accurately predictable a long time in advance, so that the mariner may carry the table of predicted Greenwich distances out with him. The plan, as we have said, was proposed more than three centuries ago, and the need of applying it was severely felt just two centuries ago. But there were no catalogues of fixed-star plans accurate enough for the purpose; and the knowledge of the moon's motions was utterly insufficient also. The double want was brought to the King's (Charles II.'s) notice, and he at once ordered an observer to be appointed and an observatory to be founded to meet it. The right man for the post turned up in the person of Flamsteed, and he was commanded forthwith to “ apply himself with the utmost care and diligence to the rectifying the tables of the motions of the heavens and the places of the fixed stars, so as to find out the so-much-desired longitude of places for the perfecting the art of navigation.” The same object is set forth in the tablet which still stands over what was the entrance-door of the building; and a wording almost identical with that just quoted has been maintained in the warrants of all succeeding Astronomers Royal to the present day.
It is desirable that this definition and its implied limitation of the duties of the Observatory should be borne in mind. There is no provision in its charter for the numerous modern subjects of inquiry which have been classed under the head of astronomical physics. It is true such subjects have been occasionally followed up, but only to an extent that precluded
* Certain corrections are necessary to reduce the angular distance as measured at the earth's surface to what it would be if measured from the earth's centre; but those form part of the after calculation.
† As a matter of fact, they are given for every third hour in our “ Nautical Almanacs.”