307 



wide marginal frill; surface with 

 three rounded tubercles, one near 

 the posterio-cardinal angle, an- 

 other, perhaps twice as large, near 

 the center of the dorsal margin, the 

 third, smaller than either, between 



Fig. 248. Beyrichia tricollina. 



and a little beneath them; addi- a left valve, with the rrm broken 



away at the anterior end, x 20 



tional slight swellings in the pos- (after uirich). 



terio-ventral fourth; moderate depression between largest 



and smallest tubercles. 



Found in the "shales of the Hamilton group, in Eighteen 

 Mile Creek, N. Y." (Ulrich, type.) 



Genus ISOCHILINA. Jones. 



[Ety. : Isos, equal ; cheilos, lip. ] 

 (1858: Can. Org. Remains, Decade 3, p. 197.) 



Carapace with equal valves, whose margins meet uni- 

 formly and do not overlap. Greatest convexity central, or 

 towards the anterior end ; anterior tubercle present. 



Isochilina (?) fabacea. Jones. (Fig. 249.) (Quart. 

 Journ. Geol. Soc, Vol. XLYL, p. 22, PL II.) 



Distinguishing Characters. — "Narrow, oblong, bean- 

 shaped, straight above, gently curved 

 below, semi-circular in front, obliquely 

 rounded behind ; faintly impressed in 

 the middle of the dorsal region ; sloping 

 gently dorsally, and more abruptly on 

 the free margins; surface apparently 

 punctate, but in reality delicately reticulate, all over." 



Found in the "Hamilton group, Eighteen Mile Creek, 

 Lake Erie Shore, N. Y." (Jones. ) 



Genus LEPERDITIA. Eonault. 



[Ety.: Lepis, scale; dittos, double.] 

 (1851: Bull. Soc. Geol. France, 2d Ser., T. VIII., p. 377.) 

 Carapace with unequal valves, the right valve the larger 

 and overlapping the left valve, along the ventral and, to 



Fig. 249. Isochilina (?) 

 fabacea. Left valve, x 25 

 (after Jones). 



