80 NEW YORK STATE MUSEUM 



width in front, while posteriorly it narrows considerably. The 

 ribs of the marginal area are finer than in the preceding species. 

 On account of the posterior convexity of the body of the valve 

 and the inequilateral development of the marginal area, the 

 entire valve has a strongly symmetric appearance. 



Dimensions. Length 2.4 mm; hight 1.7 mm; thickness .9 mm. 



Horizon and locality. Gray limestone of Rysedorph hill. 

 (Group 7) 



There is no species known to the writer which is suggested by 

 this form. If the development of the sulcus is taken into consid- 

 eration, it seems to stand at the end of a series which begins with 

 such forms as E. bulbifera, where the sulcus is deep 

 and long and begins at the cardinal line. It then begins 

 to wander ventrally, till it appears as a crescent-shaped pit only, 

 separate from the cardinal line as in E . g r a n o si a and in E . 

 s o 1 i d a ; it farther becomes faint, and may eventually disap- 

 pear entirely asin E. obliqua. At the same time the forms 

 with faintly developed sulcus have more rotund valves, 

 and mostly a convex instead of a concave marginal area; they 

 form, hence, apparently a natural group of species, which event- 

 ually may be advantageously comprised under a subgeneric term: 



primitia Jones & Holl 



Primitia mnndula Miller var. jonesi var. nov. 



Primitia mundula S. A. Miller. Cin. quar. jour. sci. 

 2:350, and Ulrich, Cin. soc. nat. hist. Jour. 1890-91. 13:132 



PL 7, fig. 2-5 



The conglomerate affords very minute representatives of the 



genus Primitia, which have been found to belong all to one 



type that probably stands in varietal relation to Primitia 



mundula S. A. Miller. 



Diagnosis. Carapace very minute, strongly convex, subquad- 

 rate-ovate, dorsal margin long, straight; anterior cardinal angle 

 obtusely rounded, posterior a little less obtuse; anterior mar- 



