138 
PALAEONTOLOGY OF NEW-YOKit, 
174. 7. ATRYPA NUCLEUS. 
Pl. XXXIII. Figs. 2 a, b, c. 
Compare A. extans ut supra. 
General form subangularly globose; length and breadth about equal ; cardinal line 
regularly curved ; beaks small, close pressed, that of the dorsal valve incurving over the 
other ; margins of the valves uniting very closely and neatly ; mesial sinus broad, flat, 
extending about two thirds the length of the shell, and produced into a linguiform extension, 
which is much elevated in front; mesial elevation prominent, rounded, extending about 
two thirds of the distance to the beak, where it is lost in the general convexity of the shell; 
surface of both valves crossed by filiform concentric lines, which are undulated in crossing 
the mesial sinus and elevation ; no visible longitudinal strife. 
This shell resembles the last one very closely, the principal difference being the extended 
cardinal line in the former ; the mesial sinus in the latter is flat and more elevated in 
front, while the mesial elevation on the opposite valve is less extant, rising more gradually 
from the surface of the shell, and not so distinctly margined by a shallow groove, nor 
produced in front. 
A comparison of a larger number of specimens may prove the existence of a gradation 
in the characters, here noticed as distinctive. 
Fig. 1 a. Ventral valve, b. Dorsal valve, c. Profile view. 
Position and locality. Middleville, Herkimer county, in the central portions of the 
Trenton limestone, associated with Conularia. 
175. 8. ATRYPA CUSPIDATAf (n. sp.). 
Pl. XXXIII.* Figs. I a, b, c, d, e,f, g, h. 
Obtusely pyramidal or somewhat obcordate ; width greater than the length ; dorsal valve 
smaller than the ventral, convex at the sides, with a broad subangular depression which 
commences near the beak, and becomes broader, deeper and more angular, elevaling the 
front of the shell into a cuspidate extension ; lower half of the sinus marked by a distinct 
narrow groove in the centre ; ventral valve very gibbous, with a broad mesial lobe which 
continues nearly to the beak, and extends in front scarcely so far as the line of the shell 
on each side ; beaks nearly equal, that of the dorsal valve incurved oyer the ventral valve, 
the latter being much more gibbous below the beak ; margins of the shell sharp ; surface 
ornamented with fine elevated concentric striae, which are imbricated towards the margin. 
The lower half of the shell, particularly on the ventral valve and sinus of the dorsal valve, 
is marked by numerous sharp radii, which gradually disappear above. 
This species bears a close resemblance to Atrypa extans ; but the specimens of that shell 
which I have been able to see, are all of separated valves, and differ from this one in the 
t Pl. xxxiii.* is a supplementary plate. 
