l.AMKU.IHKVV Ill \ 



njjht valve i.evoml that of the left. Umbonal ridge inconspicuous. Surface of shell 

 Mnooth. nothing hut an occasional growth line having been detected on any of the 

 specimens seen. A good cast of the interior shows that the pulliiil line ami muscular 

 -cars are very faintly impressed; the anterior scar is small, ovate, and situated in 

 front of the beaks close to the hinge line; the posterior scar at least twice as large 

 and situated just behi ml the center of the cardinal slope. As shown in figs. 88 and 



he relative convexity of the two valves varies, the thickness of the left in some 

 specimens being only half as great as that of the right, while in others it is quite 

 two-think A slight gap is left between the posterior edges of the valves. 



I am not acquainted with any Silurian shell with which this species might be 

 confounded. Several small species of Mmliolopsu and Colpomya </>mi.^,i are associ- 

 ate! with it, I, ut they can all be distinguished without the slightest trouble. 



formation and tooaW/jr.-Mlddk- third of the Trenton shal.-s, Chttfleld. Minnesota. 

 M*. Beg. No. 8450. 



Genus ENDODESMA, n. gen. 



Shi'll elongate, the dorsal and ventral margins subparallel, equivalved, generally 

 vciitricose. Mesial depression deep, often producing a decided oblique contraction 

 of the shell and a sinus in the basal outline. Urabones compressed, elevated consid- 

 erably above the hinge line on the anterior side, but not on the posterior side. Hinge 

 thin, apparently edentulous. A strong linear internal ligament was attached on 

 each side to a rib or ridge. Back of shell flattened or with the edges of the valves 

 bent inward without, however, forming a true escutcheon. More or less well defined 

 Innule in front of the beaks. An obscure sulcns in the middle of the cardinal slope. 

 Shell very thin; surface marked with concentric growth lines. Muscular scars and 

 pallial line so faintly impressed that they have not been determined satisfactorily. 



Type: Endodesnta ciineatunt, n. sp. 



This well marked genus is placed in the family Modiolopsidir chiefly in deference 

 to the views of Mall, Hilling*, and Meek and Worthen, who have each described a 

 species as belonging to Modiolopsis. According to my own conviction there i- little 

 indeed to suggest that genus, the shape of the shell l>eing often quite different (in 

 this respect some of the >; remind of Orthodesma) and the mesial depression 



deeper, while the faintness so far as can be seen the total absence of muscular 

 scars on casts of the interior is not only a striking but an important difference. In 

 the faintness of the muscular impressions the new genus agrees with the mn-t 

 ical forms of Arfin<nni(>i. but they an> distinguished at once by their want of a mesial 

 contraction, in consequence of which their basal outlines are gently convex instead 



