50 CLASS Ill. GASTEROPODA. ORDER IV. PLEUROBRANCHIATA. 
our present consideration, the breathing organs exhibit this arrangement 
on the right side only ; that is, but half way round the mantle, so that 
on this account he called them ‘‘ Les Semiphyllidiens.” This, however, 
as Lamarck himself acknowledged, is the only affinity between these two 
families, and therefore scarcely sufficient to establish the propriety of 
placing them together, when they differ so much in other respects. The 
Semiphyllidiana are, in fact, more nearly allied to the Aplysiana, and we 
have therefore ventured to follow Deshayes in placing them in their im- 
mediate vicinity. Their shell is smooth, being sometimes calcareous, 
sometimes horny, and either external or internal. 
The family of the Semiphyllidiana includes two genera, as follows : 
PLEUROBRANCHUS. UMBRELLA. 
PLEUROBRANCHUS, Cuvier. 
Testa interna, dorsalis, obliqué-ovata, cornea, planulata, subauriformis, 
superné convexiuscula ; vertice laterali, submarginali, subterminal, 
inflexo ; margine integro. 
Although we are indebted to Cuvier for the above generic title, as well 
as for an accurate description of the animal to which it is applied, it 
must not be forgotten that the Pleurobranchus was first noticed by our 
countryman Montague as his Bulla plumula. The attention of both these 
writers appears to have been arrested by the change and peculiarity 
of the branchiz ; the former title was selected as indicating their right- 
sided position, the latter as exhibiting their plumose structure. It was 
not until some time afterwards that the Pleurobranchi were found to be 
occasionally conchiferous, the shell in such cases being small, and merely 
of a thin horny texture, concealed within the fleshy substance of the 
mantle. 
The shell of Pleurobranchus may be described as being internal, dorsal, 
