206 
LAND AND FEESH-WATEE MOLLUSKS. 
tooth, with twelve similar laterals on each side; 
next to which is a tooth of a different form, and 
lastly, six more on each side, which latter are in 
a slight curve; the number of transverse rows 
is 75; the total number of teeth is 2,925. 
To the type belongs A. fluviatilis ; to the sub¬ 
genus, A. oblong us ; both species occur fossilized 
in the newer Tertiaries. The genus Ancylus is 
represented by one species in the Middle Eocene 
of Hordwell. 
Ancylus fluviatilis —(the River Limpet) (PI. 
XI., fig. 136).—The shell of the common “ River 
Limpet” is an elevated and regular cone, with 
the point recurved and nearer the hinder end; 
it is thin, of a yellowish-grey or horn colour, the 
inside whitish and glossy; the exterior is orna¬ 
mented with fine, close-set lines that diverge 
from the apex to the margin, but is more distinctly 
striated in the line of growth; the aperture of 
the shell is oval, and nearly a fourth of an inch 
in diameter; the height of the shell does not 
usually exceed half-an-inch. 
The shell varies somewhat in form, some ex¬ 
amples being proportionately larger and higher, 
others smaller and more swollen, and in the 
prominence of its ridges; in colour, varying 
from white to reddish-brown, and almost black. 
The shells are often coated with a brick-red fer¬ 
ruginous deposit. There is a pretty variety, with 
