DESHA YES. 599 



cannot with propriety be referred to the Nudibranches, — an 

 order so remarkable for the beauty and variety of the 

 branchial appendages with which the species are adorned. 

 " The mollusks now under consideration are, on the con- 

 trary, 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, 

 characterised by the absence of specialized branchiae ; and 

 as the function of respiration is entirely performed by the 

 skin, we propose to call this order Pellibranchiata." It 

 includes the genera Elysia or Actason, Limapontia or Cha- 

 lidis, Acteonina, and Cenia : and it is allied both to the Nu- 

 dibranches and Inferobranches, by genera which may be 

 deemed almost inosculating-. You must refer to the original 

 essay for the anatomical peculiarities that distinguish the 

 order.* 



Class ACEPHALA. 

 It is now agreed that the Heterobranches f or Tunicata 

 ought to be removed from this to form a separate class ; but 

 there is not yet such an unanimity amongst conchologists as 

 to the rank of the Brachiopoda. Cuvier considered them enti- 

 tled to a classical rank from the structure of their breathing 

 organs and the possession of their curiously ciliated arms ; 

 but Professor Owen regards them as rather an order in their 

 class. " In all essential points," he says, " the Brachiopoda 

 closely correspond with the acephalous Mollusca, and I con- 

 sider them as being intermediate to the Lamellibranchiata 

 and Tunicate orders ; not, however, possessing, so far as 

 they are at present known, distinctive characters of sufficient 

 importance to justify their being regarded as a distinct class 

 of Mollusks, but forming a separate group of equal value 

 with the Lamellibranchiata." % Deshayes, a name of great 

 authority in this point, has adopted this conclusion ; and he 

 gives us his views in a table which, it seems to me, is well 

 worth transcribing in this place. 



CONCHIFERA. 



First sub-Class. ) ( 1st sub-Order : Valves articulated. 



Brachiopoda, orh • • * • -j 

 PoLYMYARiA. j [2nd sub-Order : Valves free. 



* On a proposed new order of Gastcropodous IMollusca, by Joshua Alder 

 and Albany Hancock in Ann. and Mag. N. Hist. Scr. 2. i. 401, &c. 

 t "A name without one single quality to recommend it." — Maclkay. 

 X Zool. Trans. Lend. i. 159. 



