Fauna of the Morrow Group 
215 
to the dorsal margin, slightly sinuous a little in front of the 
middle and rounding gradually into the somewhat produced pos- 
terior outline which curves abruptly upward and forward meet- 
ing the hinge-line in an obtuse angle; beak full, elevated above 
the hinge-line and situated less than one-fourth the length of 
the cardinal line behind the anterior extremity of the valve; 
umbo tumid and distended laterally; umbonal ridge extending 
obliquely backward, merging with the posterior extension of the 
valve, the portion of the shell above the umbonal ridge moder- 
ately flattened but not so much so as the small area adjacent 
to the anterio-dorsal extremity, a distinct constriction originating 
on the umbo and extending to the ventral margin which it meets 
a little in front of the middle ; surface of valve marked by numer- 
ous, close-set, somewhat irregular concentric costae, a single 
costa varying greatly in strength as it crosses the shell giving 
a flnely rugose appearance to the surface and bearing eight or 
ten radiating costae on and above the umbonal ridge which are 
more pronounced posteriorly than near the beak. 
Dimensions of type specimen, a right valve, are : width, prob- 
ably about 15 mm. ; height, 5.5 mm. ; length of hinge-line, 14.3 
mm. ; convexity of valve, about 4 mm. 
Remarks. Although only a single not quite complete repre- 
sentative of this species has been found in the Morrow collec- 
tions, its characters are so distinctive and so well displayed that 
there is no hesitancy in describing it as a new species. It is 
distinguished from the other members of the genus by its extreme 
convexity, its surface markings, and the comparatively strong 
umbonal constriction. It is possible that the apparent depth of 
this sinus in the holotype is slightly increased by crushing of 
the valve in that region. 
Horizon and locality. Brentwood limestone: near Fayette- 
ville, Arkansas (Station 135). 
Parrallelodon cancellosus n. sp. 
Plate XV, figure 7. 
Description. Shell small, transversely sub-trapezoidal in out- 
line, very inequilateral, not quite twice as wide as high, hinge- 
line shorter than greatest width of valve below, valves moder- 
ately convex; cardinal line and ventral margin nearly straight 
