MOLLUSCA. 143 
cur on the caudal extremity of the body, as the branchiz, 
and even describes the pulmonary vein which conveys the 
blood from these to the heart. But, in his Regne Animal, 
he states it as the opinion of M. Blainville, that the fin-like 
expansions of the neck contain the branchiz on their sur- 
face, as in the case of Clio. The rectum and oviduct ter- 
minate under the right wing. Cuvier has figured and de- 
scribed the only known species, which he terms Pneumo- 
dermon Peronii, the trivial name being in honour of the 
discoverer, M. Peron. 
Posterior extremity simple. 
Genus C1i10.—Body ovate, with the tunic elongated and 
membranaceous. 
The head is divided into two lobes, the summits of which 
are furnished with tentacula. The existence of eyes has 
not been ascertained. The mouth is transverse, with two 
lateral longitudinal lips. On each side of the neck arise two 
blunt, conical, fin-like expansions, with a fine reticulated 
surface, considered as serving the double purpose of fins 
and branchie. The anus and orifice of generation termi- 
nate under the base of the right branchia. The viscera do 
not fill entirely the cavity of the inner bag. The gut makes 
only one fold. 
The genus Clio was originally instituted by Brown in 
his Natural History of Jamaica. It was afterwards em- 
braced and modified by Linneeus and Pallas, in such a man- 
ner as ultimately to exclude the species for the reception of 
which Brown originally formed it. It contains two species, 
the most remarkable of which is the Clio borealis. Mr. 
Scoresby, in his valuable work on the Arctic Regions, states, 
(vol. i. p. 544), that it occurs in vast numbers in some situ- 
ations near Spitzbergen, but is not found generally through- 
