OF DENISON UNIVERSITY, 
33 
notch rounded, and having the appearance of a flattened ear ; surface 
striae nearly obsolete on the upper half of the valves, and more dis- 
tinct on the ventral and antero-ventral regions.” 
Genus Allorisma. 
Allorisina costata, M. and w. 
(Plate III, hflg. 7, and Plate IV, Fig. 28.) 
We have not been able to compare this fine species with types, 
but in spite of its great dissimilarity, notably in size, from specimens 
referred to this species by Mr. Meek, we regard this as identical with 
the Illinois species. The generic reference is far less satisfaotory. 
Indeed, it is suspected that an examination of the hinge would prove 
this an independent genus. 
Shell medium sized, elongate ; upper and lower margins sub-par- 
allel; beak prominent, anterior; anterior margin short, convex ; shell 
produced and slightly attenuated posteriorly, posterior margin sub- 
truncate ; greatest depth of the shell a little posterior to the umbo ; sur- 
face marked with (12) strong, distant concentric cost^ extending from 
the anterior margin to the prominent posterior umbonal ridge, where 
they abruptly cease. The surface above the ridge being marked by 
fine concentric lines and a well marked ridge extending from the beak 
toward the centre of the posterior m'argin. Our specimens embrace 
impressions as well as the right valve. The space between the costae 
is gently concave and smooth and there seems to have been a radiating 
rib upon the summit of the umbonal ridge into which the concentric 
ribs merge. There are indications of an impressed line along the 
hinge line posteriorly. The outline of the posterior margin seems in 
the best preserved specimen, bi-truncate, in others obliquely truncate 
in one straight line; other minor differences are referred to the condi- 
tions of preservation. Length, 1.40; height, ,50. Comparisons were 
drawn in our note book with Orthonota, to which it may be allied. 
Allorisma Geinitzii, Meek. (?) 
(Plate IV, Fig. 27.) 
This small species resembles, somewhat, A. gilberti. White, though 
it is by no means so much produced posteriorly. Shell very small, 
