103 
(Hallotidee,) are allied to the former, but they have ROOM Xilt. 
a very expanded mouth, and no operculum; most of hist, 
these have a groove, a series of holes, or a canal, in the 
outer lip over the gills, as in the genera Pleurotomaria, 
Scissurella , Haliotis , Stomatia and Stomatella . 
The other shells of this order are simply conical, and 
not pearly; as the family of the Keyhole Limpets, 
{Emarginulidce ,) which have an animal very like the 
Earshell, but the shell is depressed, and furnished 
with a hole, placed either in the front of the apex, as in 
Fissurella , or with a nick in the front of its edge, as in 
Emarginula and Parmophorus. These holes or grooves 
afford a passage for the water to the respiratory or¬ 
gans. Here must also be placed the family of the 
tooth shells, ( Dentaliidce ,) which have been, till lately, 
regarded as the tubes of worms, but are now known 
to be formed by true Mollusca; as the tooth shell, 
Dentalium: likewise the shells of the genus Lottia , 
which are so similar to those of the Patellae , that it is 
impossible to distinguish them from each other: the 
animals, however, which form them are quite different 
from those of the latter shells, whilst they are closely- 
allied to the Fissurellre , from which they scarcely dif¬ 
fer, except in having only one gill. 
Case 49 contains the third order of Gasteropodous 
Mollusca. They have their gills placed on the right 
side of the back, and covered with a thin mantle, which 
is generally enclosed or protected by a small shell. They 
have no operculum, and usually swim about, aided by 
the membranaceous appendages on the side of the foot. 
In some Gasteropoda, the gills are on the side of the 
back, and covered by the mantle, as in the family of Bul- 
lidee. The animal of the species belonging to this family 
is divided into two portions, the head, or front part, has 
no distinct tentacula, the eyes being placed in a flat 
shield, as in the genera Bulla , Bullcea , Acer a 9 and Gader- 
optera ; the latter has no shell, and the sides of its head 
are dilated into large wings, by means of which they swim 
about in every direction. The Bullcece are peculiar for 
being 
