206 LAND AND .FRESH-WATER 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. oblongus ; 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 Iimpet) (Pl. 
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 
