156 
PALAEONTOLOGY OP NEW-YORK. 
Productella navicella. 
Productus navicella : Hall in Tenth Report on the State Cabinet, p. 172. 1857. 
Shell small, subelliptical; length greater than width : hinge-line a little 
less than the greatest width of the shell. 
Ventral valve extremely gibbous in the middle, arcuate : beak very 
much incurved and projecting a little below the hinge-line. The umbo 
is elevated above the hinge-line about equal to one-third the length of 
the shell : sides sometimes nearly parallel for one-half their length 
below the hinge-line, sometimes gradually curving outwards. Dorsal 
valve extremely concave. 
Surface marked by fine undulating concentric striae, which are some¬ 
times interrupted by irregular lamellae. On the umbo and the cardinal 
extremities there are numerous minute cylindrical spines which rise 
almost directly from the surface, or with a scarcely perceptible ele¬ 
vation of the surface just above them. Below the umbo, the ridges 
above the spine-bases become more elongate and conspicuous, while 
on the middle and sides of the valve, and towards its lower margin, 
these become elongate costae; and from them rise rounded spines, some¬ 
times interrupting the continuity of the costae, which at other times 
continue below the spine with some abatement of their strength. 
There are rarely spines rising from between the costae, and sometimes 
the latter continue for some distance without spines. 
The specimens referred to this species are distinguished by the narrow elon¬ 
gate form and extreme arcuation. No area, teeth or teeth-sockets have been 
observed, as the specimens are not in a condition to admit an examination of 
these parts. 
The typical specimen is from the Hamilton group, and I have referred to the 
same species several specimens of similar form and proportions from the Cornife- 
rous limestone, which are more distinctly marked by continuous radiating costse, 
some of which are bifurcated on the sides. All the individuals now referred to 
this species are small shells ; and though we might suppose the conditions in the 
Corniferous limestone to have been favorable to their development, yet no speci¬ 
mens have been found larger than those figured on Plate xxm. 
