142 MOLLUSCA. 
an uneven margin, narrow anteriorly, but expanding behind, 
and terminating in three projecting points. From the mid- 
dle point four ribs diverge forward, and a muscle arises, 
which, fixed in the superior viscera, enables the animal to 
withdraw into the shell. The dorsal valve is shorter than- 
the preceding, the margin flat and circular, and the middle 
convex outwardly. The branchiz are situate in the space 
between the lateral margin of the two valves, on each side, 
in a duplicature of the tunic, the sides of which are furnished 
with filaments. The fleshy neck supports the two mem- 
branaceous expansions ; between which and the base the 
mouth is situate, surrounded by two lips, and strengthened — 
within by two fleshy cheeks. The opening of the anus and 
oviduct are at the base of the right fin. 
The Hyalea tridentata, the best known species of the 
genus, was first noticed by Forskil, in his Deseriptiones Ani- 
malium, p. 124, as an Anomia, and inhabiting the Mediter- 
ranean. The same species was likewise taken in abundance 
in the Gulf of Guinea, by the expedition under Captain 
Tuckey. . 
(2.) Tunie destitute of a Shell. 
A. Fins double. 
Posterior extremity with leaf-like ridges. 
Genus PNEUMODERMON.—Head with twobundles of ten- 
tacula. 
The body is oval, with a narrow neck, and a fin on each 
side. The mouth is nearly terminal, furnished on each side 
with a fleshy lip, and beneath, with a fleshy chin. Each 
tentaculum consists of a filament, with a tubercle at the end, 
pierced by a small hole, and considered as exercising the 
office of a sucker. Cuvier, in his Mémoire sur ? Hyale et 
le Pneumoderme, considered the leaf-like ridges which oc- 
