1S6 PALEONTOLOGY OF NEW-YORK. 



Productella navicella. 



Productut navictUa : IIau in Tenth Report on the State Cabinet, p. 172. 1867. 



Shell small, Bubelliptical ; 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. ~ 



ScRFAOB marked by fine undulating concentric striaa, 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 costse ; and from them rise rounded spines, some- 

 times interrupting the continuity of the costse, which at other times 

 continue below the spine with some abatement of their strength. 

 There are rarely spines rising from between the costoe, and sometimes 

 the lattef 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 th? 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 Comife- 

 roa« limestone, which are more distinctly marked by continuous radiating costa6, 

 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 

 Comiferons limestone to have been favorable to their development, yet no speci- 

 mens have been found larger than those figured on Plato xxiii. 



