48 TRACHELIPODA. PLANARIA. 
Solarium plicatum. Plate LV. fig. 6,7. Found in the 
London Clay at Barton Cliff. 
The species of Solarium are few, and are inhabitants of tropical seas. They 
occur fossil in the Tertiary formations, and are also found as low as the 
Mountain Limestone. 
Genus XLVIII. — BIFRONTIA. — Deshayes. 
Generic Character, — Shell discoidal, planorbicular ; 
volutions disunited ; provided with a deep umbilicus, cari- 
nated at the margin; aperture longitudinal, subtriangular, 
somewhat dilated; outer lip acute, separated by a deep 
notch at both extremities. 
Bifrontia Deshayesiti, Plate VI. fig. 13. 
This is exclusively a fossil genus, the species are few, and are chiefly met 
with in the Paris Basin. 
Genus XLIX. — ORBIS. — Lea. 
Generic Character. — Shell discoidal, planorbicular, 
volutions flat, rolled upon each other, and quadrate ; deeply 
umbilicate ; aperture square. 
Orbis rotella, Plate VI. fig. 14, 15. 
This genus is known only in a fossil state, and belongs to the newer forma- 
tions of North America. 
TRIBE II.—SCALARIDES. 
Shell destitute of plice or folds on the columella ; margins 
of the aperture united in a circular form. 
Genus L.— DELPHINULA. — Lamarck. 
Generic Character.— Shell subdiscoidal, or subconic; 
solid, rugose, and umbilicated; spire small, depressed ; 
volutions few, angular, and branched ; aperture entire, 
angular, round, and sometimes trigonal, or subquadrate, 
with the sides united, and generally provided with a fringe, 
or a thick marginal ridge ; operculum horny, with numer- 
ous Close-set convolutions. 
Delphinula calcar. Plate V. fig. 16. D.conica. Plate 
V. fig. 7. Found in the Calcaire-grossier. Paris basin. 
There are but few species, and those inhabit tropical seas. 
Fossil Delphinule are found in the Tertiary formations, but are very limited 
in number. 
Genus LI. — PLANARIA. — Brown. 
Generic Character. — Shell discoidal, depressed on both 
sides ; volutions conspicuous, both above and below ; spire 
