256 MOLLUSC A. 



project from above two small cavities in the mantle. Several species inha- 

 bit the Indian Ocean. 



DiPHYLLIDIA, CuV. 



The branchiae similar to those of the Pliyllidije, but the posterior part of the 

 mantle more pointed; on each side of the semicircular head a pointed tenta- 

 culum and a slight tubercle. 



ORDER IV. 



TECTIBRANCHIATA. 



The branchias along the right side or on the back, composed of 

 laminas more or less divided, but not symmetrical; they are more 

 or less covered by the mantle, in which a small shell is generally 

 contained. They approach the Pectinibranchiata in the form of 

 the organs of respiration, and like them inhabit the Ocean. 



Pleurobranchus, Cuv. 

 The body equally overlapped by the mantle and by the foot, as if it were 

 between two shields. In some species a httle, oval, calcareous lamina is 

 contained in the mantle, and a horny one in that of others; the mantle is 

 emarginated above the head. The branchiae are attached along the right 

 side in the furrow, between the mantle and the foot, forming a series of 

 pyramids divided into triangular laminulae. The mouth, a small proboscis, 

 is surmounted by an emarginated lip and by two tubular and cleft tentacula. 

 Various species inhabit both the Mediterranean and the Atlantic, some 

 of which are large and marked with the most beautiful colours. 



Pletjrobranch^a, Meckel. 

 The branchix situated as in Pleurobranchus; but the foot projects but little, 

 and on the fore part of the former are four short, distant tentacula, forming 

 a square that reminds the observer of the anterior disk of the Acerae. There 

 is no vestige of a shell. 



Pleurob. Meckelii. The only species known; from the Mediterranean. 



Aplysia, Lin. 

 The margin of the foot turned up into flexible crests, surrounding the back, 

 and even susceptible of being reflected over it; the head supported by a 

 neck more or less long; two superior tentacula excavated like the ears of a 

 quadruped, with two flattened ones on the edge of the lower Up; the eyes 



