STREPTORHYNCHUS. t9 



The speciea described as Orthisina altemata is not distinct from this ; and al- 

 though without larger collections of specimens, it is impossible to draw lines of 

 specific distinction, yet the prevailing forms may be separated into two groups; 

 leaving a few which are not so readily referred to either of them, or which may 

 be intermediate forpis, uniting the two and likewise including S. pandora. 



I have adopted the name S. pandora as applicable to the symmetrical forms of 

 the Schoharie grit and Corniferous limestone, and some of those in the Chemung 

 group : while the S. arctostriata and iS. perversa are more characteristic of the 

 Hamilton group. 



Figs. 13 & 14 are examples of the dorsal valves of this variety. 



Fio. 15 is an irregular form of the same. 



Via. 16 is an intermediate form of the ventral valve, with the striae intermediate in distance 



and the cardinal angles bat little rounded, 

 Fia. 17 is the dorsal valve, which is quite symmetrical in form, but with distinct striss and 



rounded hinge-extremities. 

 Fio. 26 is a very convex dorsal valve with slightly rounded cardinal extremities, and having 



comparatively distinct striaa. The umbo is very prominent ; below which it is 



flattened in the middle, and slightly depressed towards the front of the shell. 



Geological formation and locality. Tl^is variety occurs on Seneca and Cayuga 

 lakes, Moscow and Hamilton, in the Hamilton group. Fig. 17 is from the same 

 group, in the village of Pabius, Onondaga county. Fig. 26 iS from the Corniferous 

 limestone at Williamsville, Erie county. In the latter, the striae are more nearly 

 equal. 



Var. D. Streptorliynchus pectinacea. 



PLATE X. 



Strophomena pectinacea and S. bifurcala ; Hail, loc. cit. 



Shells usually symmetrical, often unsymmetrical, semioval, with the 

 hinge-extremities sometimes a little salient, often truncate and some- 

 times rounded. 

 The dorsal valve is moderately convex, or sometimes gibbous. The 



surface is marked by distant, very prominent striae, with two or three 



finex ones between them. The hinge, and other features of the shell, are 



the same as in other varieties of the species. 



The well-marked specimens are usually small or of medium size. 

 [ Pal-kontolooy IV.] 10 



