134 



SCIENCE. 



[N. S. Vol. XII. No. 291. 



area. Only three species of the genus occur 

 in the Ohio Kiver. Two of these Ohio 

 River species extend west into and across 

 the Mississippi, but I know of no form be- 

 longing to the genus that is found in any 

 part of the lower 300 miles of that stream, 

 in the Pearl or the Pascagoula rivers, or 

 any of the small rivers in Mississippi or 

 Louisiana flowing into the Gulf. ]Sro mem- 

 ber of the genus is found in any of the 

 streams flowing into the Atlantic (with 

 possibly a single exception) . 



Yet the entire Alabama Eiver system is 

 filled with Pleurobemas. There are many 

 of them in the Tombigbee and Black War- 

 rior, still more in the Alabama itself, and 

 the Coosa swarms with them. But not 

 a species of Pleurobema found in the Ala- 

 bama Eiver area is identical with any found 

 in the Tennessee system. Those of the lat- 

 ter drainage area are, for the most part, 

 very closely related to each other, and be- 

 long to a single great group typified by the 

 well known Unio clavus of Lamarck. There 

 are several closely related groups of Pleu- 

 robema found in the Alabama system, and 

 all these are nearly related to the clavus 

 group, yet no member of the latter gTOup is 

 found in the southern drainage, and no 

 member of any of the southern drainage 

 groups is found in the northern drainage. 



There are a number of Uniones which 

 have a somewhat general distribution in the 

 Mississippi area including the Tennessee 

 system, that are found in the Alabama 

 Eiver drainage, such as the Unio tuberculatus, 

 of Barnes, U. ebenus Lea, U. multiplicatus 

 Lea, U. cornutus Barnes, U. pustulosus Lea, 

 U. rectus Lamarck, U. trigonus Lea, and U. 

 obliquus Lamarck. There are others which 

 are only found in the Tennessee and Ala- 

 bama systems such as U. cumberlandicus Lea, 

 U. conradicus Lea, and TJ. varieosus Lea; the 

 latter, however, extends into the Ohio 

 Eiver. 



Yet all these which occur in the Ala- 



bama and its branches have some slight 

 characters by which they differ from the 

 same species when found in the Tennessee; 

 not enough to separate them specifically or 

 perhaps varietally from each other, yet an 

 expert will generally be able to tell at a 

 glance from which system a given specimen 

 has been obtained. Unio gibbosus of Barnes, 

 an abundant, widely distributed and vari- 

 able Mississippi drainage species, is found 

 in the Alabama system, but it is shorter, 

 smaller and more humped than the type, 

 and Dr. Lea believing it to be a valid species 

 called it Unio subgibbosus. I believe that it 

 is only a variety or geographical race of U. 

 gibbosus. Unio poidsoni Conrad found in the 

 Alabama Eiver is, I am sure, only a variety 

 of the U. alatus Say, a species widely dis- 

 ti'ibuted in the central part of the United 

 States. 



A few species of Pleurobema, and certain 

 species of other genera of Unionidse closely 

 related to forms found in the Mississippi 

 valley, and evidently derived from the fauna 

 of that region are found in the Chattahoo- 

 chee, the Flint Eiver, and some of the 

 streams of Southeastern Alabama. 



In the streams draining into the Atlantic 

 from Labrador to Georgia there is found 

 everywhere a group of Unios typified by 

 Unio complanatus Dillwyn. Thei'e are a 

 great many forms belonging to this group 

 which have received specific names at the 

 hands of atithors, many of which are, ap- 

 parently, only mere variations of a few lead- 

 ing forms and not worthy of even varietal 

 names. Quite a large number of forms be- 

 longing to this group also occur in the Chat- 

 tahoochee Eiver system, some of which ap- 

 pear to differ a little from the Atlantic 

 drainage species while others do not seem 

 to be specifically different. Many of the 

 forms of this group in both the areas men- 

 tioned seem to be merely incipient species 

 and the synonymy is in a hopeless tangle. 

 Unio colujnbensis Lea, a member of the Tetra- 



