252 SHELLS AND SHELL-FISH. PART I. 



(236.) The two next types are Gasteropteron and 

 Pleurohranchus. The first is a most interesting form, 

 since it connects the tribe with the Cephalopoda. Cu- 

 vier, however, with his usual infelicity (wherever na- 

 tural affinities are concerned), simply remarks, that it 

 " appears to he an Akera, the margin of whose foot is 

 developed into broad wings for swimming, which it per- 

 forms on its back. It has no shell or stony armature to 

 the stomach ; a slight fold of the skin is the only ves- 

 tige of a branchial operculum that is visible." * A much 

 better account, however, is given by M. Rang f, who ob- 

 serves, that this '' highly interesting " animal was first 

 made known by M. Delia Chiaje, the learned anatomist 

 of Naples, who considers it so clearly a Pteropoda, that 

 he has named it Clio Amati. 



(237-) The last genus, or rather sub-family, is the 

 Pleurobranchin^, distinguished from all others by 

 their broad, flattened, and oval bodies ; but especially by 

 the feather-like structure of their gills, which are placed 

 on the right side of the body, just under the edge of the 

 mantle. Here, again, we recognise the cheloniform type 

 so frequently developed among the Testacea, and of which 

 Chelisoma, Chelinotus, Parmophorus, Chiton, Sec. are 

 such striking examples. The bodies of all the animals 

 now before us are shaped like those of tortoises ; and in 

 some, as Pleurohranchus Cuv., this resemblance is car- 

 ried so far, that the foot represents the under plate of 

 those reptiles ; so that, to use the words of Cuvier, '^ the 

 body is equally overlapped by the mantle and by the 

 foot, as if it were between two shields." The mantle, 

 in some genera, contains a calcareous plate, which in 

 Lamarck's Umbrella is so very large and strong as to 

 resemble a limpet, but is immediately known by the 

 sharp edge of its circumference. The idea that this shell 

 could by any possibility have ever been naturally attached 

 to the foot of the animal (whence the name Gastroplax), 

 is too preposterous to require further notice. We here 



* Griff. Cuv. xii. 46. f Rang, Manuel, p. 148. 



