GASTEROPODA TECT1BKANCHIATA. 245 



PLEUROBRANCHUS, Cuvier. 



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

 between two shields. In some species a little oval calcareous lamina is con- 

 tained in the mantle, and a horny one in that of others ; the mantle is emar- 

 ginated 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 lamellae. The mouth, a small proboscis, is sur- 

 mounted by an emarginated h'p, 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. 



PLEUROBRANCH^A, Meckelii. 



The branchiae 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, Linnceus. 



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 qua- 

 druped, with two flattened ones on the edge of the lower lip ; the eyes above 

 the former. The branchiae are on the back, and consist of highly complicated 

 lamellae attached to a broad membranous pedicle, covered by a small mem- 

 branous mantle, in the thickness of which is a flat and horny shell. A limpid 

 humour, secreted by a particular gland, and which in certain species is said 

 to be extremely acrid, is exuded through an orifice below on the right ; and 

 from the edges of the mantle oozes an abundant liquid of a deep purple 

 colour, with which, when in danger, the animal tinges the water for a con- 

 siderable extent. The ova are deposited in a kind of long interlaced glairy 

 net-work, of extreme tenuity. In the seas of Europe we have the 



Apl. fasciata, black, margined with lateral red crests, one of the large 

 species ; and Apl. punctata, Cuv., lilac, sprinkled with greenish points. 



DOLABELLA, Lamarck. 



The Dolabellae only differ from Aplysiae in the position of the branchiae and 

 their surrounding envelope; they are at the posterior extremity of the body, 

 which resembles a truncated cone. They are found in the Mediterranean 

 and in the Indian Ocean. 



NOTARCHUS, Cuvier. 



The lateral crests united and covering the back, a longitudinal emargination 

 excepted, that leads to the branchiae, which have no mantle to cover them, 



