KEYHOLE-SHELLS. 209 
All the species of the genus feed on marine vege- 
tables: I have watched with pleasure the little 
Grey Top (Z. cinerarius), the commonest species 
of our weed-clad rocks, rasping down with the 
teeth of its ribbon-lke tongue the minute Con- 
ferve that grow on the inside of my glass vases. 
With a pocket lens it is easy to see the process, 
which I can compare to nothing else than the mode 
in which a cow licks up, as it were, the grass, as she 
moves along, by successive sweeps of her tongue. 
FAMILY FIsSURELLADZ. 
(Keyhole-shelis.) 
In external form and appearance, the shells 
which compose this group bear the closest resem- 
blance to the Limpets (Patellade). All of them, 
however, have the peculiarity of an orifice in the 
shell, either at the summit of the cone, or in the 
form of a slit at the front edge. 
The characters of the included animals dis- 
tinguish them at once; they have well-developed 
heads, with short muzzles, and tapering tentacles, 
at the outer bases of which the eyes stand on short 
footstalks. Beneath the mantle on each side is a 
series of short tentacular filaments, similar in cha- 
racter to those of the Zvochide. ‘There are two 
gill-plumes, which are large, pectinated, and equal; 
they are placed in an ample cavity, which commu- 
nicates with the aperture of the shell, whether 
this be situated on the summit, or in the front 
margin. 
All the mollusks of this family are marine, and 
are distributed through the seas of most parts of 
the world; but principally those of warm climates. 
P 
