178 PALAEONTOLOGY OF NEW-YORK. 



papillose or papillose-striate, without evidence, or with very slight evi- 

 dence of any inequalities of the exterior. (A few gentle pustule-like ele- 

 vations have been observed in one specimen.) The cardinal margin is 

 wrinkled, and the extremities rouqded. There is a short bilobed cardinal 

 process and a slender mesial septum, which reaches half the length of 

 the shell. The occlusor muscular impressions are distinctly visible; and. 

 from between them, on each side, proceeds the reniform vascular impres- 

 sion, which is at first slightly inclined forward, and, then curving down- 

 wards about half the length of the valve, is again shortly recurved, ter- 

 minating in a broader expansion, about halfway from the mesial septum 

 to the lateral margin of the valve. 



lu the specimens examined, this is a neat and well-marked species. The ventral 

 valve, when having few spines, somewhat resembles the young of P. rarispina; 

 but it is a more delicate shell, with more slender spines. The shell is thin, easily 

 exfoliated, and presenting a very finely striate surface beneath, or on the partially 

 exfoliated portions. The greater proportion of individuals have the surface pretty 

 evenly studded with spines. 



The dorsal valves associated in the same specimens of the rock with the ven- 

 tral valves are presumed to belong to the same species, and are known by their 

 abrupt deflection towards the front, the finely papillose-striate surface, and ab- 

 sence of pustules or nodes indicating fossets or spine-bases on the exterior surface. 



Geological formation and localities. In some semicalcareous bands in the Che- 

 mung group at Cherry creek, Chautauqua county; and at New- Albion, Cone- 

 wango and Coldspring in Cattdraugus county. New- York. 



Prodactella liystilcula (n. s.). 



PLATE XXVL 



Shell small, concavo-convex, semielliptical : hinge-line rarely equalling 

 and usually somewhat less than the width of the shell. 



Ventral valve gibbous or ventricose, somewhat hemispheric, regularly 

 arching, sometimes a little flattened in the middle, abruptly depressed 

 towards the cardinal extrcynities, and produced in small scarcely 

 defined ears. The umbo is a little elevated above the hinge-line ; the 

 apex is often perceptibly truncated, and there is an extremely narrow 

 area. 



