LAMELLIBRANCHIATA. 527 
Endodesma orthonotum.] 
ENDODESMA POSTLATUM, 2. 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 usa clue to 
the later development of the genus. As may be seen from the figures the species is 
closely related to EF. cuneatum but, as the name implies, the posterior width (hight) 
is greater in L/. postlatum. Ina 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 Modio- 
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 Hndodesma. 
Formation and locality.—Uppermost beds of the Galena, Dubuque. Lowa, where if was collected by 
Mr. Charles Schuchert. 
Mus. Reg. No. 8345. 
EnpoprEsMa ortHoNoTUM Meek and Worthen. 
PLATE XXXVII, FIGS. land 2. 
Modiolopsis orthonota MEEK and WorTHEN, 1868, Geol. Sur. LIl., vol. iii, p. 295. 
Modiolopsis rectiformis WORTHEN, 1882, Bull. No. 1, Ill. 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 
