88 
NATURAL HISTORY. [[UPPER FLOOR. 
or a canal in the outer lip over the gills, as in the genera 
Pleurotomaria, Scissurella , Haliotis, Stomatia and Sfo- 
matella. 
The other shells of this order are simply conical, and 
not pearly ; as the family of the Keyhole Limpets, ( Emar - 
gimdidce,) which have an animal very like the Ear-shell, 
but the shell is depressed, and furnished with a hole, 
placed either in the front of the apex, as in Fissureila, or 
with a notch in the front of its edge, as in Emarginula and 
Parmophorus. These holes or grooves afford a passage for 
the water to the respiratory organs. 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, JDentalium : likewise the shells of the genus 
Lottla, which are so similar to those of the Patella ?, 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 Fissurellce, from which they scarcely differ, 
except in having only one gill. 
Case 49 contains the third order of Gasteropodous Mol¬ 
lusca. 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 oper¬ 
culum , and usually swim about, aided by the membrana¬ 
ceous 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- 
lidce. The head or front part of the animals belonging to 
this family has no distinct tentacula, the eyes being placed 
in a flat shield, as in the genera Bulla , Bullcea, Accra, and 
Gasteroplera ; the latter has no shell, and the sides of its 
head are dilated into large wings, by means of which it 
swims about in every direction. The Bullcece are peculiar 
for being provided with a hard shelly gizzard. The family 
of Sea Hares, (Aplysiadte,) so called from the form they 
assume when sitting on the rocks, have an elongated- head 
and distinct tentacula, as the genera Aplysia, Dolabella, 
and Notarckus; the latter has no shell. The Syphonarice 
have an external conical shell, with a groove in the side 
