MUSEUM OF COMPARATIVE ZOOLOGY. 309 
truncation about as much as the posterior cardinal margin falls toward the 
upper angle of the same; truncation vertical, but hardly angulated; a faint 
ridge running from the beaks to the upper corner, more marked in the smaller 
valve; sculpture of fine not very regular concentric undulations, coinciding 
with the lines of growth; surface finely granular, but appearing nearly smooth; 
interior polished; the sinus deep and rounded; margins very thin, smooth, and 
plain; not interrupted under the beaks; ossicle bridge-shaped, wide, short, 
concave behind in the middle line, very large for the size of the shell; points 
of insertion not perceptibly raised; extreme length of shell 6.25; length of 
anterior part 4.75; max. alt. 6.0; diameter 2.75 mm., of which the deeper 
valve takes about 1.50 mm. 
Habitat. West of Florida in 17 fms., one specimen; Marco, Florida, in 2 
fms., H. Hemphill. 
This unpretending little shell has the aspect of a very young Periploma or 
Thracia. It is only when the interior is examined that its peculiarities become 
manifest. It is possible that, among the innumerable fossil genera or sections 
which have been proposed, some one may eventually turn out to include the 
present form, but none of those I have been able to examine agree with it. 
Corimya Agassiz is perhaps the nearest, but has submedian beaks with one or 
two internal ribs, the posterior cardinal margin slightly thickened as if for an 
external ligament, and there is no evidence of a pallial sinus, or rather the po- 
sition of the posterior adductor scar would indicate that there was no sinus. 
The Californian species differs from the Floridian in having the beaks less 
posterior, and, though a larger shell, in having a more slender and delicate 
ossicle, which resembles, in some sort, a very long-winged butterfly with its 
Wings spread. 
It is interesting to add to the links connecting the East and West American 
faunz, and pleasant in so doing to be able to clear up the obscurity which has 
somewhat interfered with the relegation of this genus to its proper place in the 
system. 
Suspcenus BUSHIA Dat. 
Shell inequivalve, inequilateral, truncated behind, but not gaping; porcel- 
lanous; concentrically sculptured; hinge toothless, with a large U-shaped 
ossiculum fitting in the apices of the beaks, which are filled with solid shelly 
matter; a strong external ligament, but for which the hinge-line is not bent or 
thickened. 
This differs from the typical Asthenotherus in its porcellanous, instead of 
earthy, shell-substance, destitute of granulations; its completely closed valves; 
in possessing an external ligament ; and in the filling of the apices of the beaks 
with a solid shelly mass on which the extraordinarily large and strong arched 
ossiculum is planted, as on two pedestals. 
It is dedicated to Miss Katharine J. Bush, of New Haven, whose excellent 
work on Mollusca I have had frequent occasion to refer to. 
