ORTHIDES OF THE HAMILTON GROUP. 
49 
area scarcely more than half as wide as the ventral area, slightly in¬ 
clined inwards. Yentral valve gibbous towards the umbo, depressed- 
convex in the centre and flattened towards the front; the front margin 
straight, or without sinus; area less than a line in width; foramen 
very broad, nearly twice as wide as high; beak slightly incurved and 
neatly pointed. 
Surface marked by fine, radiating, bifurcating striae, which are crossed 
by finer concentric striae, and by more distant subimbricating lamellose 
lines of growth. Minute tubular openings are observed upon the surface 
of the striae in nearly all conditions of preservation, sometimes giving 
a subimbricated appearance to the lines. Besides these, well preserved 
specimens show minute pores or puncta, distinct from the punctate 
structure oflHhe shell; while the shell about the margins of these ap¬ 
pears to have been extended, to form slender seta, the bases of which 
are sometimes preserved. In weathered specimens, or in those where 
the striae have been partially dissolved by the decomposition of iron 
pyrites, these characters do not appear. 
The interior of the dorsal valve shows a prominent cardinal process, 
which is continued in a strong median ridge for about half the length of 
the valve, below which are sometimes seen a few vascular markings. The 
impressions of the adductor muscles are, rarely, faintly separated by a 
transverse undefined elevation. 
The muscular imprint in the ventral valve, which extends for two- 
thirds or more of its length, is somewhat broadly oval, flabellate, and 
deeply marked by the adductor muscular scar. The external striae usual¬ 
ly mark the inner margins of the valves, and, in young shells or thin 
specimens, are visible as far as the muscular impression. 
This species is closely allied to the 0. vanuxemi , and may perhaps prove only 
a .variety of that species. In authentic specimens the shape is more ovate, the 
cardinal extremities less rounded, and the sides sloping almost directly to near 
the middle of the shell; the dorsal valve is more gibbous, and the umbo and 
area are more inclined inwards, as maybe seen by comparing the profiles of figs. 
3 e and 3 k, Plate vi, with a corresponding figure 3 e on Plate vii. The yentral 
[ Paleontology TV.] 7 
