ROOM XIII.] 
NATURAL HISTORY. 
91 
Pleurotomaria, Scissurella, Halioiis, Stomaiia and Slo- 
matella. 
The other shells of this order are simply conical,, and 
not pearly ; as the family of the Keyhole Limpets, ( Emar - 
ginulidce,) 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 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 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, Dentaliwn likewise the shells of the genus 
Lottia , which are so similar to those of the Patellce, 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 Fissurellcv , 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 Bid - 
lidce . 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, Bullosa, Acera, and Gasteroplera ; 
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 Bulloeoe are peculiar for being pro¬ 
vided with a hard shelly gizzard. The family of Sea Hares, 
(.Aplysiadoe ,) 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 Notar- 
chus ; the latter has no shell. The species of the family 
of the Syp/ionarice have an external conical shell, with a 
groove in the side for the passage of the water to the gills. 
