LAMELLIBEANCHIATA. 527 



Endodesma orthonotum.T 



Endodesma postlatum, n. sp. 



PLATE XXXVII, FIGS. 5 and 6. 



Of this species I have seen only a single imperfect specimen — which -under 

 ordinary circumstances would scarcely merit description. Being however the most 

 recent existence of the genus now known it is of interest as it may give us a clue to 

 the later development of the genus. As may be seen from the figures the species is 

 closely related to E. cuneatum but, as the name implies, the posterior width (hight) 

 is greater in E. postlatum. . In a cardinal view both ends also are more obtuse, giving 

 greater convexity to the sides; the umbonal ridge, though, prominent, is not so 

 sharply rounded, the umbones fuller, the anterior end shorter, and the posterior 

 margin more broadly rounded and most prominent in the basal half instead of near 

 the middle. Finally, the mesial sulcus crosses the valves more obliquely and the 

 sinuation of the ventral margin is wider and nearer the' center of the length of the 

 shell. 



The side view of this shell is considerably like that of several species of Modlo- 

 lopsis, but the absence of a strong anterior muscular scar on casts and the presence 

 of the concave areas bordering the hinge line proves that it is not a Modiolopsis but 

 an Endodesma. 



Formation and toeaHiy.— Uppermost beds of the Galena, Dubuque. Iowa, where it was collected by 

 Mr. Charles Schuchert. 



Mus. Beg. No. 8345. 



Endodesma orthonotum Meek and Worthen. 



PLA.TE XXXyil, PIGS. 1 and 2. 



Modiolopsis orthonota Meek and Woethen, 1868, Geol, Sur. 111., vol. Hi, p. 295. 

 Modiolopsis rectiformis Worthen, 1882, Bull. No. 1, 111. St. Mus. Nat. Hist., p. 38. 



Shell elongate, the length being nearly three times the hight; valves quite 

 convex, the greatest convexity being a short distance above the middle, in front of 

 which they have an undefined concavity commencing in the umbonal region and 

 widening and deepening to the base in front of the middle. Cardinal margin long, 

 very nearly straight or but slightly arched; posterior margin obliquely substrunca- 

 ted, sometimes very faintly sinuous above, and rather narrowly rounded below the 

 middle; basal margin subparallel to the dorsal, gently convex behind the middle, 

 and broadly sinuous between the middle and the front; anterior side short, con- 

 tracted beneath the beaks, narrowly rounded. Beaks depressed, appearing on a line 

 with the dorsal margin, strongly incurved, placed about one-sixth of the entire 

 length of the valves behind the anterior extremity; lunule rather large but not 



