A STUDY OF THE UNIONIDE OF ARKANSAS, WITH INCIDENTAL REFERENCE TO THEIR DISTRIBUTION IN THE MISSISSIPPI VAL

LEY.

BY R. ELLSWORTH CALL, M. Sc., M. A., M. D.

There exists very little published information concerning the abundance, varieties, and geographic distribution of the great molluscan family of Unionidæ, within the limits of the State of Arkansas. A few forms were originally accredited to it; some of these have not since been found in the State, nor have some others ever occurred outside its boundary. Of the mollusks of no other one State in the Union is less known.

During the progress of the investigation the results of which are herein recorded, opportunity to consult a number of original types has been afforded with some very interesting results connected with the nomenclature of this great group of mollusks. To those who have not had access to original publications and to original specimens, much of the synonymy herein developed will, perhaps, prove a matter of surprise. The task which one thus assumes is not without its share of responsibility. To do strict justice to those most excellent early investigators, who so completely have rendered all successive students their debtors, is no easy task. That each one did the best he could, or knew, we scarcely can doubt; measured by the faulty notions of species that prevailed during their time they could, perhaps, for the most part, have done little better. A half century spent upon this great family by the veteran Dr. Isaac Lea must, in itself, entitle his opinions to confidence and to weight. But authority may never stand in place of Nature and in place of fact. If so be a more modern notion of what constitutes specific value, coupled with proper regard for environmental factors,

has necessitated the reduction to synonymic rank of very many forms it is only because the facts justify the reduction, and the interests of sound scientific reasoning require it. Of so much synonymy as grew out of personal differences between former students it were better not to speak. It will always stand as a reproach that the best interests of science have thus suffered.

Most of the opinions herein expressed, regarding the specific value of very many forms, are based upon an extensive private collection of Unionida, which is geographically and numerically all but complete. Added thereto are very many facts gleaned during an engagement of some months in the Smithsonian Institution, at Washington, the time of which was largely devoted to the Unionida, which had previously been studied by my old friend and preceptor, the competent and painstaking Dr. James Lewis. Many facts, chiefly, however, connected with geographical distribution, were thus collated.

A considerable number of American Unionida, most of which are represented in the fauna of Arkansas, were originally described by the great French naturalist, Lamarck. Concerning these species there has been much difference of opinion, and even yet, in certain cases, opinions are divergent. To facilitate a correct understanding of Lamarck's species his original descriptions are herein reproduced, and fuller and more complete descriptions added, of forms which are believed to be authentic. These descriptions are accompanied by drawings in the several plates; some of these were executed by the careful hand of Mr. Harry A. Pilsbry and are duly accredited to him on the plates; the remainder were drawn by the writer. Added to the data thus assembled will be found, for Lamarck's species, most of the synonyms which have been erected as species upon his older described forms. The determination of these synonyms was in no sense a patriotic matter, but proceeded on the hard lines recognized in science as just and right. The great naturalist made serious errors, but these could not well be avoided with scanty material and not too full locality referIt is hoped that this rather full synonymy,— which is

ences.

not complete, we too well know, will eliminate from the trade-lists of amateurs species names which should no longer burden our faunal lists. In the matter of other bibliographic references the rule adopted has been to give the volume, page, plate, figure and date reference where the form was first described; added to these are occasional references to wellknown and easily accessible works. There has been made no attempt, in the case of the greater number of forms listed, to exhibit but a moiety of the bibliographic matter. Beginning with Unio elegans Lea, references have been made to Reeve's great work, the Conchologia Iconica; this has been done in the hope that those to whom access is given to that work will find the critical notes, which are sometimes appended, of service. Many American shells are therein wrongly named but the fault lies in the sending abroad of misnamed specimens of American Unionida. Some of these errors are corrected by Reeve in the addenda to the Genus Unio; the rest remain to puzzle the naturalist. The opportunity to examine and use this great and costly work was afforded by the generous courtesy of Mr. Truman H. Aldrich, of Cincinnati, who kindly loaned me these volumes for a long period of time.

Not the least interesting fact connected with the study of the Unionidæ is the one that numerous species have been duplicated by describing the forms assumed by the sexes as of specific value. This has occurred in a number of instances, and is responsible for a considerable number of synonymic names. Among the forms so described, the following may stand as examples: —

Unio donaciformis Lea is the female of Unio zigzag, Lea. Unio ater Lea is a synonym of Unio purpuratus Lamarck, and is based on the female form.

Unio patulus Lea is based on the female of Unio clavus Lamarck.

Unio lens Lea and Unio leibii are both synonyms of Unio circulus Lea and are based on the female form. Unio leibii

is a dwarfed form of U. circulus.

Unio brevidens Lea is a male form of a species the female of which was afterwards described as Unio arcæformis Lea.

Unio subovatus Lea was based on the male of Say's Unio ovatus.

This list, illustrating what appears to have been the misinterpretation of sexual differences, might be extended indefinitely; when extended to its limits the student of the Unionida will be astonished at the results which he will reach.

Aside from personal collections, made in the intervals of field work in geology, and which were made in the St. Francis, Ouachita and Saline rivers, help has been derived from specimens collected by Professor R. T. Hill, in the Ouachita, by Mr. L. S. Griswold, in the Ouachita well up toward its source, and by Mr. F. A. Sampson, in the White river and in other portions of northern Arkansas. Dr. John C. Branner, State Geologist, has furnished an occasional specimen. Other shells have been sent, for identification, from the Little Red river. It is a matter of regret that more full and exhaustive collections could not be made preliminary to a final paper; it would better represent the wealth of the State in this group of natural objects. It will be noticed that three streams and as many localities furnish the major portion of the species here listed. Reasoning from this fact it is fair to assume that very many forms yet remain to be added to the list on complete examination of the State.

REGISTER OF SPECIES.

UNIO ABERTI Conrad.

Proc. Phila. Acad. Nat. Sci., p. 10, (1850); Jour. Phila. Acad. Nat. Sci., 2d series, Vol. II, Plate XXVI, Fig. 1, (1851).

Unio lamarckianus Lea. Trans. Am. Philos. Soc., 2d series, Vol. 10, Pl. 17, Fig. 20, (1852).

Unio popenoi Call. Bull. Washburn Coll. Lab'y of Nat. Hist., No. II, pp. 48-49, Pl. II, (1885).

This form was described by Lea from the Caddo river, under the name of Unio lamarckianus, the specimens of which were submitted to him by Dr. Byrd Powell. Additional examples were submitted by Dr. Hale who collected them in the Ouachita river, near the Hot Springs. The specimen figured by Doctor

Lea is a young one and is by no means a fair illustration of the shell. In the description of the species he mentions the numerous small nodules found over its surface but the figure shows the shell as smooth. The very young shells are nearly as triangular as the well known Unio elegans Lea.

In 1885 the writer, without then having access to the complete bibliography of the species, and misled by the great size of the specimens submitted to him, described the form as new, giving it the name of its Kansas discoverer. Later the error was discovered by him and the facts fully stated.* In this last named paper the remarkable character of the ctenidium was made known and illustrated from specimens collected in the Verdigris river, Kansas, by Mr. J. R. Mead.

This species has thus far been only found in the Arkansas and Red river drainage basins. It has not occurred to us in our collecting in the State.

UNIO ALATUS Say.

Nicholson's Encyc., Am. ed., Vol. IV, Pl. IV, Fig. 2, 1816. Also figured in the Am. Jour. of Science and Arts, 1st series, Vol. XIV, Fig. 17a and 176. Another good figure may be found in Conrad's Monograph of Unio, Plate XXXI. A figure has recently appeared, in Bull. U. S. Fish Commission, Vol. XIII, Pl. 36, 1893, that is characteristic in all respects, except its alate features. Both alæ are broken in the specimen figured.

This species has not been found abundantly in Arkansas. Its sole occurrence to us was in the St. Francis river, near Wittsburg, in Cross county. It has been seen by the writer from the Ouachita river, Indian Territory, and without doubt will be found in the Arkansas portion of that stream. It is not readily confounded with any other known Unio being, when perfect, easily separated from other symphynote species by its dark purple coloration within and its flattened disk. I have received it under the name of Unio purpuratus from which species, however, it is entirely distinct.

From the Cedar river, Iowa, were secured very large and

* See American Naturalist, September, Vol. xxi, 1887, p. 860.