SHUMARDELLA 



225 



lete, from two to five, more usually two or three, originate on or in front 

 of the umbonal region and pass to the anterior margin along the bottom 

 of the sinus, these plications are low and rounded, somewhat variable 

 in strength but always comparatively faint, upon the lateral slopes of 







FIG. 19. A series of eight cross-sections of the rostral portion of the shell of 

 Shumardella obsolens (X 2%), showing the internal characters. 



the valve the plications are obsolete. Internally the hinge-teeth are 

 supported by short dental plates which scarcely extend beyond the articu- 

 lation of the hinge. 



Brachial valve gibbous, the ^umbonal region protuberant, sometimes 

 extending nearly as far as the beak of the opposite valve, the greatest 

 convexity near the anterior margin, the surface strongly convex from 

 beak to front along the median line, and much more strongly convex 

 transversely, towards the beak the surface on each side is a little in- 

 flected to form, with a similar inflected portion of the opposite valve, a 

 small, more or less conspicuous concave lateral region ; mesial fold obscure 

 or obsolete in the posterior half of the valve, becoming a little more con- 

 spicuous anteriorly, but rarely or never strongly differentiated from the 

 general surface of the valve ; the beak incurved beneath that of the 

 opposite valve; plications obscure as in the opposite valve, the stronger 

 ones are confined to the mesial fold where they alternate with those of 

 the sinus of the opposite valve, upon each lateral slope two or three 

 exceedingly faint plications may occasionally be detected, but they are 

 more apt to be entirely obsolete. Internally there is no cardinal process ; 

 the hinge-plate is divided but not to the base at the apex of the valve, 

 it is supported posteriorly by a median septum which is simple at first 

 but is divided above anteriorly, each division supporting one side of the 

 hinge-plate, the support of the hinge-plate by the divided septum does 

 not continue beyond the median incision of the hinge-plate itself so that 

 no open crural cavity occurs, but rather an enclosed cavity roofed 



8 



