164 PALAEONTOLOGY OF NEW-YORK. 



become elongated below, with nnmerous oblique wrinkles. It is equally difficult 

 to unite these with typical forms of P. shvmardiana, though ultimately all these 

 varieties may be found to constitute only phases of a very variable species. 

 Some examples of dorsal valves, similar to the typical forms of P. exanthamata, 

 have been found in the Chemung group ; but all these are moderately concave, 

 and never approach the form of typical P. shumardiana. 



Geological formation and localities. In the shales of the Hamilton group, on the 

 shore of Seneca lake ; and at Tinker's falls, Onondaga county. 



Productella tuUia ( n. s.). 



PLATE XXIII. 



Shell broadly ovate, subhemispheric ; hinge-line equalling or a little 

 less than the greatest width of the shell. 



Ventral valve ventricose with the apex extremely incurved, and much 

 producfid in front. 



Dorsal valve more or less gibbous, often moderately concave in the 

 upper and middle portions, and abruptly, deflected and produced in 

 front. 



Surface of ventral valve marked by scattered slender spines. Specimens 

 of the dorsal valves are nearly all of the interior, strongly wrinkled 

 on the hinge-line, with numerous pustuliform elevations, which be- 

 come elongated on the front of the valve. Between these stronger eleva- 

 tions are numerous fine and sometimes coarser radiating striae which 

 become more distinctly visible as the shell is exfoliated from the 

 inside, showing that they pertain to the exterior surface. The cardinal 



<-' process is bifurcate, and of considerable length. 



The interior of the dorsal valvo is recognized by numerous pustuliform ridges 

 and distinct intermediate striae, the latter a character not observed in any other 

 species. 



The fignres 41-44 illustrate the usual form and features of this species as it occurs in the 

 Hamilton shales. 



Geological formation and localities. In the Hamilton group, near Tully and 

 Delphi falls in Onondaga county. A single dorsal valve of larger size and mode- 

 rate convexity is from the Tully limestone. 



