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1461 



TERTIARY FAUNA OF FLORIDA 



restricted group, which is already named by Ferussac, we are thrown back 

 upon P. fontinale C. Pfeiffer (not Cyclas fontinalis Drap.), which is practically 

 in the same case, so we are obliged to follow what has been the common prac- 

 tice and accept P. amnicum as type, unless we are prepared to drop the name 

 Pisidium altogether. Following up the other names, we find Pera (Leach 

 MS.) Jenyns (in synonymy) to be an absolute synonym of Corneocyclas. 

 Euglesia (Leach MS.) Gray, 1840 (in Turton), and Gray, 1847; Pisum Gray, 

 1847, Cordula Leach, 1852, and Fluminina Clessin, 1873, are all based on Tel- 

 Una amnica Miiller, and are therefore synonyms of Pisidium s. s. Galileja 

 Costa and Euglesa (Leach, 1852) Clessin, 1873, are not to be separated from 

 Corneocyclas. Cycladina Clessin, 1871, founded on P. obtusale, is identical. 

 Rivulina Clessin was based on P. globulare Clessin (=P. pusillum Jeffreys, 

 1862, not Gmelin), which does not seem to be distinguished from Pisidium s. s. 

 by any valid characters. Fossarina Clessin (P. Henslozvianum Sheppard) 

 is preoccupied in Gastropoda since 1863 by A. Adams. Having thus cleared 

 the track of some obstructions, it remains to revise the classification, which 

 has been of a mechanical and not a truly morphologic character hitherto. I 

 have in this work accepted Westerlund's determination of the species, many 

 of which, fortunately, are represented in the National Museum by author's 

 specimens sent originally to Doctor Gwyn Jeffreys, and purchased with his 

 collection. As the arrangement by Clessin, based on minor conchological 

 features, must be rejected, and most of his groups are not only heterogeneous 

 but absurd, his names must follow their typical species and, as we have seen, 

 they will fall into synonymy. 



There can be no doubt, I think, that the most fundamental characters in 

 a group which is so uniform in externals, and which in hinge characters finds 

 itself arrested before the nepionic characters are wholly lost, are those of the 

 nepionic shell as preserved on the umbones of the adult, and in the present 

 revision I shall adopt this hypothesis. 



The nepionic shells of Corneocyclas present marked characters, enabling us 

 to give clear definitions of the various groups. They may be simply convex 

 and concentrically striated, passing without any strongly marked change of 

 characters into the disk of the adult valve, or the stage when the change took 

 place may be marked by a slight but evident sulcus, constriction, or line of 

 limitation. In another group the nepionic valves are flat and exhibit one or 

 two relatively broad, stout undulations, which, when the ventral edge of the 

 nepionic valve and the undulation coincide, appears in profile as a raised edge 

 or keel on the convexity of the disk. A third type has on the nepionic surface 



