POTAMIS. TRACHELIPODA. 43 
spirally grooved ; aperture subquadrate, or ovate, its upper 
part modified within by the abdominal region of the body, 
with the peritreme a little thickened, and sometimes broadly 
reflected, and a groove at its upper extremity ; columella 
arcuated, with a sharp spiral plica at its base, and forming 
the upper margin of the canal, which is somewhat short, 
truncated, and generally reflected ; provided with a small 
horny operculum, 
Cerithium pyramidalis. Plate IV. fig. 1. Found in the 
London Clay at Barton Cliff and Hordwell. 
The species are very numerous, and subject to great variety in form; they 
inhabit the seas of almost all countries. The following are the principal 
peculiarities which they exhibit. 
1. Outer lip considerably dilated and reflected, prolonged over the canal at 
the anterior extremity, and connected to the termination of the columella ; 
producing in the not reflected canal a circular aperture, with a sharp margin ; 
external surface ribbed and decussated. C. sulcatum. 
2. Canal very short, with a prominent callosity at the posterior region of 
the inner lip. C. tuberculatum. 
3. Spire very short; aperture almost circular, and extremely short; the 
canal not protruding ; surface gronose. C. breviculum. 
4. Apex decollated in the adult shell; outer lip greatly dilated and re- 
flected ; almost no canal, the aperture nearly quite orbicular ; outer surface 
longitudinally ribbed. C. decollatum. 
5. Outer lip thickened, but not reflected ; hardly any canal; with numerous 
varices. (C. varicosum. 
6. Posterior part of inner lip with a callosity ; canal almost straight, and 
somewhat elongated; one varix opposite to the aperture; outer surface 
Trugose. C. mutatum, 
7. Outer lip hardly thickened; inner lip thickened and prolonged; canal 
elongated and reflected ; columella often with a plait in its centre. C. 
columna. 
8. Canal wide, very short, both sides of equal length; outer lip dilated to- 
wards the anterior part, and externally provided with one row of tubercles. 
C. Pacificum. 
The Cerithia seem intermediate between the genera Terebra and Planaxis, 
they are, however, distinguished from the latter by having a canal, of which 
it is destitute, and from Terebra by the form of the columella, and reflected 
canal ; and although somewhat related to Pivena, they will at once be recog- 
nized, by the back part of their outer lip being entire. 
Fossil species are very numerous, and occur in the Blue Marls, south of 
France, the Super Cretaceous rocks of Bordeaux, and Dax, the Cretaceous 
and Oolitic groups of rocks. 
Genus XXX VIII. — POTAMIS. — Brongniart. 
Generic Character. — Shell turreted ; body short; spire 
long ; aperture nearly semicircular, but destitute of a canal 
in the upper angle ; base contracted into a short, slightly 
truncated beak ; outer lip dilated ; operculum corneous, 
