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 Ixxv 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. Ix.w, figs. 11, 12; pi. xcv, fig. 30. 



