TRENTON CONGLOMERATE OF RYSEDORPH HILL 73 



projecting " obliquely outward, the apex extending beyond the 

 ventral margin and curving towards the anterior extremity of 

 the valve; the section of the spine at the base is elliptical, becom- 

 ing sharply angular on the posterior side as it nears the apex." 

 The shape and relation of this spine seems to be subject to some 

 variation,, as in the largest specimen found and figured (pi. 7, fig. 

 20) it is short and clawlike, while some younger ones possess 

 a remarkably long and straight spine (pi. 7, fig. 24). This is 

 always situated in the posterior half and directed posteriorly, the 

 anterior half being marked by its lesser convexity and the often 

 very distinct eye tubercle. If the tubercle shown in the figure 

 of Walcott's type specimen also represents the eye tubercle, the 

 spine is directed backward in that form also. The surface of 

 the specimens from Rysedorph hill is smooth and shining; the 

 muscle spot has not been observed. 



A feature not mentioned in the description of the type, but 

 indicated in the drawing and of great prominence in the writer's 

 material, is a depression, extending medially from the middle of 

 the cardinal line to the ventral margin. In internal casts of this 

 thick-shelled ostracod it appears as a very deep furrow slightly 

 curved ventrally. A shorter impression corresponding to the 

 nuchal furrow is sometimes observed on the anterior half; it 

 does not, however, contain the eye tubercle, which lies farther 

 forward. 



On account of the absence of any observable overlap, the 

 leperditoid form, the shallow median depression and the distinct- 

 ness of the eye tubercle and muscle spot, this form is better 

 retained under the genuis' Isochilina, and is probably not related 

 to other genera with a similar median depression such as Leper- 

 ditella and Primitiella. 



Horizon and locality. Black and reddish gray compact lime- 

 stones of the Rysedorph hill conglomerate. (Groups 5, 6) 



