142 MOLLL'SCA. 



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 branchiae 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 Forskal, in his Descriptions 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.) Tunic destitute of a Shell. 



A. Fins double. 



Posterior extremity with leaf-ltike ridges. 



GENUS PNEUMODERMON* Head with two bundles 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 Memoire sur FHyale et 

 le Pneumoderme, considered the leaf-like ridges which oc- 



