of Gasteropodous Mollusca. 405 



an order, so remarkable for the beauty and variety of the branchial 

 appendages with which the species arc adorned. The mollusks 

 now under consideration are, on the contrary, distinguished by 

 the extreme simplicity of external form, and by the absence of 

 any specialized breathing organs. It would, therefore, be more 

 in conformity with the views on which the existing orders of 

 Gasteropoda were established by Cuvier, to consider this group 

 as forming a separate order, chai'acterized by the absence of spe- 

 cialized branchiae ; and as the function of respiration is entirely 

 performed by the skin, we propose to call this order Pelli- 

 branchiata, and to include in it the following genera : — 



Elysia, Risso (Actaon, Oken). 



Limapontia, Johnston (Chalidis, Quatrefages) . 



Acteonia, Quatrefages. 



Cenia, Alder and Hancock. 



The Placobranchus of Van Hasselt, a genus involved in great 

 obscurity, may possibly belong to this order, as it has a very 

 evident relationship with Elysia ; but, as it is stated to have la- 

 mellated branchiae, disposed on the back and lateral lobes, we 

 think it more probable that it is an aberrant group of the Nudi- 

 branchiata, forming a passage to the present order through the 

 genus Elysia, the latter being itself a slight departure from the 

 more simple form of the Pellibranchiata. 



On the other hand, this order is nearly allied to the Infero- 

 branchiata through a small mollusk which we have already de- 

 scribed in the ' Annals of Natural History*/ having very much 

 the form and appearance of the Pellibranchiata, but possessing 

 external plumose branchiae, under the right side of the cloak, as 

 in the former order, to which it must consequently be referred. 

 This animal we take to belong to the genus Pelta of Quatrefages, 

 though the characters he assigns to that genus differ in many 

 respects from those we find in our species. 



In consequence of the extreme degradation from the molluscan 

 type that M. de Quatrefages has stated to exist in these little ani- 

 mals, they have become objects of much interest in a physiolo- 

 gical point of view, and we therefore consider ourselves fortunate 

 in having the opportunity of examining two or three of the ge- 

 nera, and especially in having got one of these in such great 

 abundance, as, notwithstanding its minuteness, to enable us to 

 make out its anatomy very satisfactorily. The anatomy of this spe- 

 cies [Limapontia nigra) we now purpose giving in detail, and from 

 the slight examination we have been able to make of the other two 

 genera, species of which we have described, we believe it may be 

 taken as a fair example of the anatomical characters of the order ; 



* Vol. xviii. p. 289. 



