PLEUROBRANCHUS. 267 



side of the body is furnished with a branchial plume, from 

 which circumstance the family derives its name. The heart is 

 non-symmetrical, with one auricle, and lies on the right side 

 anterior, and in contact with the branchial plume; and its 

 hennaphroditism being modified by congression, none of the 

 Patelloid genera can receive it. The Bullida and Aplysiada 

 reject the genus, from the very different disposition of the 

 branchiae ; neither can it be deposited in Lamellaria, in which 

 we have placed the L. perspicua of Montagu, which, though 

 somewhat similar in outward appearance, is an animal closely 

 allied to the Muricidce. It appears then that this aberrant 

 group must be intercalated in the main line where the least 

 violence is done to natural order ; it is therefore fixed imme- 

 diately after the Patelloid forms, as first showing that advanced 

 system of reproduction termed hennaphroditism with mutual 

 congression. 



PLEUROBRANCHUS, Cuvier. 



P. MEMBRANACEUS, Montagu. 



P.membranaceus, Brit. Moll. iii. p. 558, pi. 114. f. 5; (animal) 



pl.X.X. f. 3. 

 Lamellaria membranacea, Montagu. 



Animal thick, placed between two large subcircular disks ; 

 the ventral one, which is the foot, is symmetrical, and in the 

 specimen observed, when fully extended was 4 inches in 

 diameter ; its colour underneath pale yellow, marked with a 

 multitude of irregular anastomosing deeper yellow lines. The 

 dorsal disk is strong and coriaceous, and an inch less in 

 diameter than the ventral one; it is nearly symmetrical, 

 having a sinuosity posteally and anteally, with, at about its 

 centre, imbedded in the skin, a very thin, wrinkled, mem- 

 branous, blush-coloured auriform shield, exhibiting a metallic 

 lustre ; its posterior extremity is subspiral, and its use is for 

 the protection of the viscera, but not of the branchial plume, 

 which is without its range ; the upper disk is studded with 

 numerous large and smaller yellow prominent papillae, and in 



