OLINTOM SROTTP. 



63 



group, and finally reaches its maximum development at the period of the Delthyris shaly lime- 

 stone. In the Onondaga and Corniferous limestones it is still quite frequent, but has assumed 

 in some degree other characters. 



The figure in the Geological Report of the Third District, page 79, was drawn from a 

 Niagara specimen, and I have never seen specimens in the Clinton group so perfect and well 

 developed. 



Position mid localities. This species occurs in the lower calcareous beds of the group ; in 

 the shaly sandstones associated with the ore beds in Kirkland ; in the fucoidal beds near Clin- 

 ton and other places ; in the Pentamerus limestone at Rochester, and in the higher shale and 

 limestone of the group at numerous localities farther west. ( state Collection.) 



New Genus STROPHEODONT A. 

 [Gr. (frpoipsvs, cardo, and o5ou^, dens.'\ 



Shell with the general form and characters of Lept^na (viz. one valve convex and the other 

 concave, the concave one following the same curve, and nearly parallel to the convex ^ne) ; 

 cardinal area continuous, nearly linear, mostly occupied by the dorsal valve, striated trans- 

 versely ; foramen decidedly closed ; ventral valve with the hinge-line uninterrupted ; margins 

 of the hinge-line crenulated ; area striated very strongly in the transverse, and more slightly 

 in the longitudinal direction*. Muscular impressions somewhat bilateral. 



The crenulated hinge-line is a very strong distinctive character, since in Lept^na this 

 margin is smooth. In true Lept^na, also, the area is striated only longitudinally (that is in 

 the direction of the hinge-line), and the foramen is in part occupied by a projection of the 

 ventral valve which fills it ; while in Stropheodonta, the foramen, if it ever existed, is entirely 

 closed by the growth of the dorsal valve, and the hinge-line of the ventral valve is straight 

 and continuous. 



The striae of the shell in many species of Stropheodonta differ from those of Lept^na, and 

 some of the species are readily distinguished by this character alone. 



449. 1. STROPHEODONTA PRISCA (n. sp.). 



Pl. XXI. Fig. 9 a, h. 



Shell semioval, wider than long, apparently contracted at the extremity of the hinge-line ; 

 hinge-margin crenulated ; striae fine, closely arranged. 



The specimen is an imperfect valve, nearly flat, with the hinge-line distinctly crenulated ; 

 but it is interesting as being the earliest known species of this type, appearing at the same 

 period with Chonetes. The specimen figured is a cast, or rather impression of the inner side 

 of the concave ventral valve, and the impression left in the stone is consequently concave. It is 



* The Leptcena demissa of Conrad (Hamilton group), may be regarded as the type of this genus. 



