° 
182 S. W. Ford—The Genus Oboleila. 
in my possession vary in length and breadth from one and one- 
half to six lines, the two diameters being generally nearly 
equal. The surface of both valves, when perfect, is both radi 
ately and concentrically striated. The shell is thick and solid, 
showing no tendency to break up into successive lamin on 
weathering. I have had portions of it ground and polished for 
microscopic examination, but am unable tu make out any defi- 
nite structure. 
much thickened, the several scars bounding the elevation. 
The interior surface of the forward portion of the valve is 
marked by fine radiating strize. 
The dorsal valve possesses a small though distinct area, 
which is divided into two equal portions by a feeble longitu- 
dinal ridge. The slender sardtial Hire is delicately notched in 
the middle, and has immediately in advance of it a deep trans- 
verse groove (fig. 2,4). On either side of the longitudinal ridge 
referred to, there is a small, ovate, cardinal muscular sear. These 
scars have their apices directed downward and outward, their 
upper portions cutting across the extremities of and limiting the 
cardinal line. Directly in front of the cardinals there are two 
larger impressions of similar shape and direction, the laterals, 
which extend forward to the mid-length of the shell. These 
two pairs of impressions are frequently connected with each 
other, by the cardinals passing down into the laterals; but, as 
will be seen, they are not so connected in the specimen figured, 
which has been selected in order to illustrate more clearly their 
essential independence. In the central portion of the valve 
there is a pair of still larger impressions le c), having their up- 
per portions parallel, and their lower, falcate parts, widely 
diverging. Between their parallel portions there is a !oW 
mesial ridge, which dies out before reaching the hinge-line. 
The falciform portions of these scars are, in general, very 
faintly impressed, and might readily escape observation. The 
interior surface is usually smooth. 
