482 PHILOSOPHY OF ZOOLOGY. 



This genus differs from Aplysia, in the dorsal plate being 

 calcareous and hard. The fore part of the body is narrow ; 

 behind, it is larger, and obliquely truncated. The disc thus 

 formed is circular, surrounded with a fringe of fleshy fila- 

 ments. From the centre of this disc, a longitudinal slit ex- 

 tends forward, a little way beyond the anterior margin, and 

 contains the branchiae. The position and structure of the 

 other organs are precisely similar to those of the Aplysia. 



This genus was instituted by LAMARCK, from characters 

 derived exclusively from the dorsal plate or shell. CUVIEII 

 afterwards examined a species brought from the Mauriti- 

 us by PERON, which he considers as the one figured by 

 RUMPHIUS in his Amboinshe Rariteitkamer, Tab. x. No. 6., 

 and which he has consecrated to his memory, naming it 

 Dolabella Rumphii. 



&. Tentacula two in number. 



49. PLEUROBRANCHUS. Cloak and foot expanded, be- 

 tween which, on the middle of the right side, the branchiae 

 are placed. The cloak is strengthened in the middle above 

 the- branchiae by a thin expanded subspiral shell. The neck 

 is short, and in some contracted, with the front emarginate, 

 exhibiting the commencement of the inferior tentacula. The 

 upper tentacula are tubular and cloven. The gills occur at 

 the edge of the dorsal plate. In front of these are the ori- 

 fices of the organs of generation, and the anus is situate 

 immediately behind the gills. 



The mouth is furnished with a short retractile, proboscis. 

 The tongue occupies both sides of the mouth, and is covered 

 with spines. The gullet is enlarged into a kind of crop be- 

 fore it enters the stomach : this is folded, and is divided by 

 contractions into three parts. The first stomach has muscular 

 walls of moderate thickness, with a single longitudinal band. 

 The second has membranaceous walls, with longitudinal 

 internal ridges, and the third has thin and simple walls. 



