xl 
SYNOPSIS OF GENERA. 
expression of most of the species of this genus is very characteristic, and there 
is little difficulty 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 
Chemung 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. lxxv, 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)A 
[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 description of this genus whicli 1 find is the following, from a footnote on the page above cited : 
“ It is a small, smooth, compressed, elongated, eqnivalve bivalve, with nearly parallel, straight upper and 
lower margins, and a distinct, rather large, rectangular notch in the anterior ventral margin, forming a 
hiatus similar to that seen in the genus Xylopliaga, thoug-h it evidently has no relation to that group, but 
seems to be allied to the Solenidse. In Dr. Hayden’s Report on the Geology of Nebraska, I have proposed 
for this genus the name Prothyris .” The Report on Nebraska, page 223, contains the following: “ Prothyris, 
Meek, 1869, Proc. Acad. Nat. Sci. Phila., July, p. 172.” 
