239 
PALAEONTOLOGY OP NEW-YORK. 
Ambocaifa praumbona. 
PLATE XL1Y. 
Orthis prceumbona : Hall, Tenth Report on the State Cabinet, p. 167. 1857. 
Jlmboccelia prceumbona : Hall, Thirteenth Report on the State Cabinet cited, p. 71. 1860. 
Shell obliquely semiglobose or ovoid, the hinge-line less than the width 
of the shell;-cardinal extremities rounded : surface without plications. 
Ventral valve extremely convex or ventricose, with a shallow impressed 
line which has nearly the character of a narrow shallow sinus : beak 
large and incurved; area of moderate dimensions, proportionally high, 
with the lateral margins not defined; fissure partially covered by an 
arching pseudo-deltidium. 
Dorsal valve moderately and evenly convex, usually without mesial fold 
or sinus, the cardinal extremities rounded. Sometimes there is a narrow 
impressed line down the centre of the valve. 
Surface essentially smooth, or marked only by concentric striae, which 
are sometimes crowded into imbricating folds. In rare examples, I have 
observed towards the margin a kind of punctate marking, which does 
not belong to the texture of the shell, and may indicate the bases of 
seta; but the evidence is unsatisfactory. 
Little is known of the interior of the valves. The cast of the ventral 
valve has a large often pustulose muscular area, margined by a thick¬ 
ening of the shell; without evidence of dental plates. The dorsal valve 
has a bilobed cardinal process, with the bases of the crura extended along 
the inner surface of the valve. 
In many specimens of this shell the length and width are nearly equal, the 
dimensions being about three-fourths of an inch. In some individuals there is an 
extreme elongation of the ventral valve, as shown in figure 5 a of Plate xliv. Its 
greater size and absence of mesial sinus, and proportionally shorter hinge-line, 
distinguish this species from Ambocodia umbonata.. 
Geological formation and localities. This species occurs in the soft shales of the 
Hamilton group, on the shores of Seneca and Cayuga lakes, and upon Canandaigua 
lake. Though sometimes abundant in certain layers in a few localities, it is far from, 
having the wide geographical range of the Ambocodia umbonata. 
