MIOCENE MOLLUSC A AND CKUSTACEA. 41 



Mr. F. B. Meek, in his revision of the genus Barbatia, 1 further describes 

 and characterizes the genus as follows: " Shell transversely oblong-ovate, 

 with rounded extremities and a deeply sinuous basal margin; surface radi- 

 ately striate; area moderate; cartilage furrows very small, crowded, and 

 crossing the area at right angles to the cardinal margin; hinge rather dis- 

 tinctly arched, with denticles hollow." 



There have been so many generic divisions of this group of shells that 

 it has become extremely difficult to say to which of several any certain 

 form must properly belong, and in the present instance it is somewhat diffi- 

 cult to say wherein the present genus differs from some others; particularly 

 Breviarca of the same author, except in the hollow teeth-like denticles. 

 Mr. Meek ranges Striarca as a Section under Barbatia of Gray, and Mr. 

 Try on ranges Breviarca as a section under Lamarck's genus Cucullaea; on 

 what grounds, however, they are to be so separated I do not quite see, as 

 Striarca centenaria, the typical species, has the muscular impressions elevated 

 and margined to a much greater extent than any Breviarca I have ever 

 seen. In general form, perhaps, there may be some more reason for the sep- 

 aration than in the internal structure, as in the present genus the body of 

 the shell is sulcated along the middle, but as Conrad states its basal margin 

 is "closed," not having a "small gape," as Mr. Tryon supposes, while that 

 of Breviarca is full and rounded. Still, I do not think this ought to be con- 

 sidered as of generic importance. Perhaps the hollow teeth in this case 

 might safely be taken as of generic importance, but so far as I know it is a 

 species by itself yet, no other one having been recognized as possessing this 

 peculiarity. So far, however, it would only be a specific feature, and no 

 general feature distinguishing the two can readily be found. 



1 Invert. Paleont., vol. 9, U. S. Geol. Surv. of the Terr., p. 78. 



