807 



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 tricoUina. 



and a little beneath them; addi- a left vaiye, with the rriii broken 



away at the anterior end, x 30 



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



terio-ventral fourth; moderate depression between largest 



and smallest tubercles. 



P'ound in the "shales of the Hamilton grouj), in Eighteen 

 Mile Greek, N. Y." (Uirich, type.) 



Genus ISOCHILINA. Joxes. 



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

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



Carapace with e(|ual valves, whose margins meet uni- 

 formly and do not overlap. (Greatest convexity central, or 

 towards the anterior end ; anterior tubercle present. 



IsocHiLixA (?) FABACEA. Jones. (Fig. 249.) (Quart. 

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



Distinguishing Characters. — "Narrow, oblong, bean- 

 shaped, straight above, gently curved 

 below, semi-circular in front, obli(piely 

 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 gi'oup, Eighteen Mile Creek, 

 Lake Erie Shore. X. Y." (Jones. ) 



Fig. 249. Isochilina (.?) 

 fabacea. Left valve, x 25 

 (after Jones). 



Genus LEPERDITIA. Ronault. 



[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 



