140 
TROCHUS extensus, 
TAB, CCLXXVIII .—Figs. 2 and 3 . 
Spec. Char. Depressed, conical ; above rugged, 
obliquely striated ; beneath convex, smooth ; 
base expanded, with a broad undulated thin 
margin ; aperture oblong ; umbilicus not pli- 
cated, closed when old. 
Nearly twice as wide as high: the upper part of each 
whorl is irregularly undulated, and elevated in the middle 
into an angular ridge, which is more or less conspicuous 
in different individuals : the umbilicus is large and nearly 
smooth : it is sometimes covered over. 
This Trochus in many respects resembles Trochus 
Benettise* ; but although I have many specimens, none 
of them have attached shells or stones to themselves : 
one indeed has two or three oysters adhering to it ; but 
from their position over the spire, it is evident that they 
were not picked up by the Trochus. The umbilicus 
distinguishes the species. 
Fig. 2 represents a specimen filled with Pyrites, 
found in the Cliff at the Isle of Sheppy, The remaining 
figures were found in the Highgate Tunnel in 1811 . 
they are filled with indurated Marl : we do not know 
how far they may extend their shells, but the lower 
figure seems extravagant. 
* Voi. I. p. 224. tab. 98 , 
