SYNOPSIS OF GENERA . 
liii 
Cytherodon, Hall (Twenty-third Report on the State Museum of Natural 
History. Generic illustrations, pi. xiv, fig. 21. 1872). 
[Type, Cytherodon nasutus, Hall.] 
Shell equivalve, inequilateral, form rhomboid-ovate, gibbous; anterior end 
short, declining almost vertically from the beaks and rounded below into the 
basal margin. Cardinal line arcuate. Umbonal slope gibbous, not defined. 
Post-cardinal slope concave. 
Surface marked by fine concentric striae of growth which become lamellose 
below and posteriorly. 
Hinge area strong, with several angular teeth or crenulations beneath and 
anterior to the beaks. Posterior to the beaks the hinge-line is marked by a 
deep ligamental groove. Anterior muscular scars distinct, just within the mar¬ 
gin of the shell; posterior one large and shallow, situated on the post-cardinal 
slope. Pallial line simple. 
The general form of this shell is similar to some species of Schizodus, but the 
umbonal slope is less defined, the muscular impressions are not so strong, and 
are differently situated. The hinge characters are entirely distinctive. Before 
having learned the hinge structure of the American forms of the latter genus 
I referred some of them to Cytherodon in the belief that they were generi- 
cally identical. A further study of these forms has shown the hinge teeth and 
muscular impressions corresponding to Schizodus. These differences may be 
seen by comparing figs. 1, 2 and 3 with figs. 11 and 12 of plate lxxv and with 
fig. 30 of plate xcv. 
The typical form is known in the Hamilton group of Maryland and New 
York. 
Type and example: Cytherodon nasutus , pi. lxxv, figs. 11, 12; pi. xcv, fig. 30. 
