xl SYNOPSIS OF G ENEMA. 



expression of most of the species of this genus is very characteristic, and there 

 is little ilifficulty in assigning them to their proper generic association. 



At the present time we do not know of any species of the genus below the 

 horizon of the Upper Helderberg group. They are more numerous in the 

 Clienmng than in the Hamilton group, and several species have been observed 

 in the Waverly sandstone. The latter have a more extended and narrower 

 posterior end than the Chemung forms. 



American examples : Schizodus tumidus, pi. xv, figs. 25-29. 



Schizodus appressus, pi. Ixxv, figs. 3-9 ; and other species 

 of the same plate of this volume. 



Prothyris, Meek (Proc. Acad. Sci. Phila., vol. xxi, p. 172. August, 1869).* 



[Type, Prothyris elegans, Meek.] 



Shell equivalve, inequilateral, extremely elongate ; cardinal and basal mar- 

 gins often sub-parallel ; anterior end rounded or sub-truncate, with a deep notch 

 in the antero-ventral margin. Posterior end rounded, lanceolate, or truncate. 

 Cardinal line straight or slightly arcuate. Cardinal slope sometimes sub-alate. 

 Umbonal slope rounded and undefined or sub-angular. 



Surface of the shell marked by fine concentric striae of growth, and in one 

 species described, by fine radiating striae on the cardinal slope. Internal char- 

 acters unknown. 



Two species of this peculiar generic form have heretofore been described. 

 The type is from the coal measures, and the other from the Waverly sandstone. 

 Four additional species are illustrated in the present volume, two of which are 

 from the Hamilton and two from the Chemung group ; one of the latter pre- 

 senting some peculiarities. The shells of this genus are thin and fragile, pre- 



•The only <lt>8criptioii of thiHg-enus which I tiiul is the following, fi-om a footnote on the page above cited : 

 " It in a Hinall, Hm(M)th, compresseii, elongateii. equivalve bivalve, with nearly parallel, straight upper and 

 lower inargioH, and a distinct, rather Urge, rectangular notch in the aiit<!rior ventral margin, forming a 

 hiatus similar lo that seen in the genus XyUrphaga, though it evidently hius no relation to that group, but 

 Sfpms to \<e allied to the Solenidre. In Dr. Hayilen's Report on the Geology of Nebi-aska, I have proposed 

 for this genus the name ProViyris." The ReiMtrt on Nel)ra8ka, page 223, contains the following : " Pi-othyris, 

 Meek. 1869, Proc. Aca<l. Nat. Sci. Phila., July, p. 172." 



