ORTHIDES OF THE HAMILTON GROUP. 
47 
B. Species of the type of Orthis hybrida and Orthis oblata. 
Orthis vanuxemi, Orthis leucosia, Orthis penelope, Orthis cyclas. 
Orthis vairaxemi, 
PLATE VI. 
Orthis vanuxemi ; Hall, Tenth Report on the State Cabinet, 1857, p. 136. 
Shell subcircular or transversely suboval, compressed : hinge-line very 
short; margins of the valves crenulated within from the external striae; 
interior minutely punctate. Dorsal valve convex : beak scarcely dis¬ 
tinct from the cardinal border, not incurved; cardinal process promi¬ 
nent ; area flat or slightly inclined to the ventral area, and about two- 
thirds as wide. Ventral valve nearly flat or a little concave towards the 
front, moderately convex in the umbonal region : beak small, extending 
little beyond the opposite beak, arched, and rarely incurved over the 
area ; area very small, less than half the greatest breadth of the shell, 
arcuate ; foramen comparatively large, triangular, and partly filled by 
the cardinal process of the other valve : teeth prominent. 
Surface marked by fine, closely arranged, radiating tubular striae, which 
are perforate at intervals, increasing both by implantation and bi¬ 
furcation, and are crossed by very fine indistinct concentric striae, and, 
at greater intervals, by more distinct, concentric, imbricating lines of 
growth : entire surface granulate or punctate, under a magnifier. Striae 
from twelve to sixteen in the space of two lines near the beak, and 
from seven to nine in the same interval near the margin. 
The interior of the dorsal valve shows a. strong cardinal process, which 
is continued in a prominent rounded median ridge for half the length of 
the shell, where it sometimes divides, or gradually becomes obsolete : 
there are sometimes visible low transverse ridges, which divide the 
muscular impression. The crural processes are prominent, and sustained 
below by strong oblique ridges. In the interior of young specimens, the 
marks of the external strise are visible nearly or quite to the muscular 
impression; while in older specimens these marks extend little beyond 
the margin. 
