LAMELLIBRANCHIATA. 525 



Eododesma.] 



right valve beyond that of the left. Umbonal ridge inconspicuous. Surface of shell 

 smooth, nothing but an occasional growth line having been detected on any of the 

 specimens seen. A good cast of the interior shows that the pallial line and muscular 

 scars 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 sitrfated just behind the center of the cardinal slope. As shown in figs. 33 and 

 35, the 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-thirds. 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 Modiolopsis and Colpomya demissa are associ- 

 ated with it, but they can all be distinguished without the slightest trou"ble. 



Formation and loealitt/.— Middle third of the Trenton shales, Chatfleld, Minnesota. 

 Mus. Beg. TSo. 8450. / 



Genus ENDODESMA, n. gen. 



Shell elongate, the dorsal and ventral margins subparallel, equivalved, generally 

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

 of the shell and a sinus in the basal outline. TJmbones 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 

 lunule in front of the beaks. An obscure sulcus 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: Endodesma cuneatum, n. sp. 



This well marked genus is placed in the family Modiolopsidce chiefiy in deference 

 to the views of Hall, Billings, and Meek and Worthen, who have each described a 

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

 indeed to suggest that genus, the shape of the shell being often quite different (in 

 this respect some of the species 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 most typ- 

 ical forms ot Actinomya,hxitthej Sire distinguished at once by their want of a mesial 

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



