II St. Maurice and CIvAiborne Pei^ecypoda ii 



Conrad describes tlie one valve of this species as radians : Oblong, 

 compressed, lobed and flexuous on one side ; the ribs numerous, radiated ; 

 beaks very small, not prominent, pointed and a little curved laterallj-, 

 Vance's Ferry, S. C; Claiborne, Ala. 



O. sellcBforrnis : Oblong, convex, thick and ponderous, lobed; one 

 side of the larger valve profoundly sinuous and the opposite side gibbous ; 

 smaller valve sinuous and a little convex ; dorsal margin long and slightly 

 arched, with both extremities obtusely rounded. 



Since these are but different valves of the same species, Con- 

 rad 's intimation that there are two valves of his sellceforuiis was 

 based on an error. This error, however, he corrected in 1842, 

 strangely choosing sellcsformis instead of radians, a more appro- 

 priate name perhaps, and coming first in his original publication. 



As species are most generally defined in modern literature 

 several of the sellceforinis-Y\\LQ forms here included under one 

 name might be regarded as distinct species. 



With a small amount of material at hand, from a limited 

 number of localities, perhaps no one would hesitate to affirm that 

 vQxy well defined species could be differentiated. But with the 

 increase of material from intermediate localities the seeming dis- 

 tinctness of certain types rapidly disappears. Nevertheless there 

 is considerable fi.xitj^ to some of these varietal forms and it is 

 well worth while to have them defined and named and ha^-e 

 their interrelations and stratigraphic significance pointed out. 



0. seilaeformis var. smithvillensis, Plate 3 



From Smithville, Tex., are derived numerous small speci- 

 mens with thin shells, and rarely, larger representatives, show- 

 ing one plicate valve and one nearly smooth valve, giving a 

 somewhat compressirostra appearance. Yet the plications are 

 very numerous and without the foliations and squamose charac- 

 ters of the latter species. The flat valve shows signs of weak pli- 

 cations. There is an entire lack of a marginal kink or fold — the 

 feature doubtless first suggesting the name sellcBforinis. 



0. sellseformis (tj^pical) Pis. 6 and 7. 



Rarely in Texas do the large, ponderous, saddle-formed mu- 

 tations of this species occur. From near Columbus on the Sabine, 



