30 TRACHELIPODA. PTEROCERAS. 
TRIBE IV.— ALATA. 
Shell provided with a canal of greater or less extent at the 
base of the aperture ; the right lp changes its form as the 
animal progresses towards maturity, and is provided with a 
sinus towards its base. 
Genus XXIII. — STROMBUS.— Linneus. 
Generic Character. — Shell oblong, subventricose ; spire 
more or less acutely turreted ; aperture generally elongated, 
and in most species with a short canal at its superior extre- 
mity, and in some cases having an elongated narrow canal, 
extending nearly to the apex of the spire; outer lip sharp 
edged, and entire, in the young state, but dilating and 
thickening with age into an expanded wing, lobate at its 
superior edge; sometimes with several notches both above 
and below; but having a well marked sinus near the infe- 
rior extremity or base; this is sometimes very strong, and 
in other instances almost obsolete ; aperture provided with 
a long, narrow, horny operculum. 
Strombus laevis. Plate III. fig. 22. Found at Grignon. 
S. pugilis. Plate X. fig. 8. Found in the Blue Marls, 
south of France. 
The Strombi inhabit the seas of warm climates. There are many species. 
Fossil species occur but seldom ; they are met with in the newer formations, 
above the chalk, in the Calcaire-grossiér, Paris, and in a similar formation 
at Bordeaux. The tertiary Traps at Vicenza, contain two or three species, 
Genus XXIV.— PTEROCERAS, — Lamarck. 
Generic Character. — Shell subturreted, oblong-ovate 
subventricose ; spire short; body very large, its base ter- 
minating in an elongated, generally recurved pointed canal ; 
aperture oblong, its superior extremity extended into a 
canal, which is sometimes donble; outer lip thin, acute at 
the edge when young, but, in the adult state, thickened, and 
dilated into a wing-shape, its margin provided with a series 
of digitations or horns, and with a deep sinus near the lower 
extremity, but not close to the base; outside usually tuber- 
culated, coated by a thin, horny epidermis ; operculum 
horny, thick, oblong, rounded above, and pointed below, 
One or two species have a single horn-shaped process in- 
serted between the basal sinus and the lower canal. 
Pteroceras Oceani. Plate VI. fig. 9. Found in the 
Kimmeridge Clay at Havre and the Jura. 
