31 * 
PALAEONTOLOGY OF NEW-YOEK. 
Genus Atrypa (Dalman). 
SPIRIGERINA (d’Oebignt) . 
The Genus Atrypa was founded by Dalman in 1827, to include a group 
of palaeozoic shells which he described as “ inequivolve, 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 
Linnaeus 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 sinall round perforation, whic? 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 
sometimes 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 
the fossil. 
