319 PALEONTOLOGY OF NEW-YORK. 



Genus Atrita (Dalman). 



SPIRIGEHINA (d'Orbiobt). 



The Genus Atrtpa was founded by Dalman in 1827, to include a group 

 of paloBozoic shells which he described as " inequivalve, biconvex, hinge-line 

 "rounded, beak of larger valve covering the base of smaller valve, apex 

 " imperforate." 



Under this genus the two first named species are A. reticularis of 

 LiNN^us and A. aspera of Schlotheim, both of which have a perforation 

 in the apex of the ventral valve, though it is often concealed by the 

 curvature of the beak. Notwithstanding that the name in its signification 

 is a misnomer, it has been very generally adopted, and for a long time 

 it was used by some authors to include species usually referred to Tere- 

 BRATULA, but which have only a remote relation to that genus, and are 

 now distributed under various generic designations. 



Restricting the signification of the term Atrypa to forms congeneric 

 with those above referred to, we have a well-defined and strongly marked 

 group of shells which may be characterized as follows : 



Shells suborbicular, transverse or elongated; valves articulating by 

 teeth and sockets : beak of the ventral valve produced and incurved; 

 the apex truncated by a small round perforation, which is sometimes 

 separated from the hinge-line by a deltidium ; a false area in some 

 forms well-defined, but often not existing in the same species.* This 

 valve is more or less convex or nearly flat, sometimes with a broad and 

 well-defined sinus, and often with a scarcely perceptible depression. 

 Dorsal valve convex, often extremely gibbous; with or without a 

 mesial fold. 



• This false area is formed by the thickening of the shell in the bottom and sides in the upper part, 

 and the final filling up of the rostral cavity above the line of the teeth ; the pedicel-groove being 

 Bometimes visible to the base of the area, but there is scarcely evidence of its passage beneath it, and 

 the apex is apparently solid. The condition of the shells examined, however, is such that there possi- 

 bly may have been a minute foramen in the living shell, which has been closed by mineralization in 

 th« fossil. 



