76 . -MONOMYARIA. SPHERULITES. 
One species only is known in a recent state, namely, the Hipponyx mitratus, 
which inhabits the West Indian Seas. Several fossil species have been detected 
in the Calcaire-grossier of France. 
The H. mitratus was, until lately, considered a univalve, and has been — 
figured and described as Patella mitrata, The fossil species have been also in- 
eluded by Lamarck among his Patelle. 
Genus XVI.— CALCEOLA. — Lamarck. 
Generic Character. — Shell equilateral, inequivalve, tri- 
angular; umbones separated by a large, depressed, irregu- 
larly and transversely striated, trigonal area in the lower 
valve, which is the larger of the two, and very deep, funnel- 
shaped, and obliquely truncated at its upper side; hinge 
margin transversely straight, linear, notched, and slightly 
toothed in the centre ; the upper edge arcuated ; upper or 
smaller valve semiorbicular, semicircularly striated, and 
serving as a lid to the lower valve; internal cardinal edge 
furnished with two lateral tubercles, a central pit and smaller 
plate. 
Calceola Sandalina, Plate IX. figs. 2, 3. 
The very peculiar form, and other characteristics, of the Calceola, distin- 
guishes it from all other bivalves; namely, its great thickness and solidity, its 
being striated internally, from the centre to the circumference, and being des- 
titute of a ligament. In general form it has much the appearance of a lady’s 
slipper. It is supposed to have been attached by the beak or umbo of the 
larger valve. 
There is but one species of this genus, and it is fossil, found in the neighbour- 
hood of Juliers in the Mountain Limestone. The only fossil to which the 
Calceola bears the slightest resemblance is spirifer cuspidatus. Plate VII. 
figs. 8, 9, 10. 
Genus XVII.— RADIOLITES. — Lamarck. 
Generic Character. — Shell inequivalve, and interiorly 
striated, radiating from the inside of the umbones; both 
valves somewhat turbinated and conical, the lower one 
larger than the upper, which has the appearance of a did. 
Radiolites ventricosus. Plate VIII. fig. 3. 
Known only in a fossil state, and it is even doubtful whether the Radiolites 
are really testaceous shells. 
Genus XVIII]. — SPHAZRULITES. — Lamarck. 
Generic Character. — Shell greatly inequivalve, orbicular 
or suborbicular, two valves when united forming a double 
cone; upper valve lid-shaped, and generally depressed 
above, with two unequal tubercles within; under valve 
cup-shaped, subventricose, with scales radiating beyond the 
i 
