160 Prof. G, J. AUman on the Anatomy 0/ Actseon. 



described it (Mem. de la Soc. d^Hist. Nat.) as a Nucleohranch 

 nearly allied to Firola, and there is at least as much reason for 

 considering it in this point of view as for assigning it a place 

 among the Nudibranchs. Rejecting therefore Pterosoma from 

 the Nudibranchs as a moUusk as yet too imperfectly understood, 

 the remaining four families are judiciously constructed, based as 

 they are upon true differences of organization, and consisting of 

 really natural groups. ^ 



In the beautiful work of Messrs. Alder and Hancock on the 

 British Nudibranchiate MoUusca, the first part of which, just 

 published by the Ray Society, I have had an opportunity of 

 seeing, since the present paper was placed in the printer^s hands, 

 these gentlemen distribute the British genera under the three 

 families, Dorididce, Tritoniadce and Eolididce. They make more- 

 over a most important revision of the old genus Tritonia, separa- 

 ting from it the Tritonia arborescens of previous authors, which 

 they find, notwithstanding its divided branchiae and general Tri- 

 ^^tonia-like appearance, to possess a true Eolidian structure, and 

 'which they accordingly locate in the family of the Eolidida as a 

 distinct genus under the name of Dendronotus. 



In order however that Actceon may also find a place among the 

 MoUusca Nudibranchiata, an additional family must be formed" 

 for its reception. The family which it is thus found necessary to 

 constitute will perhaps correspond with the Placobranches of 

 Sander Bang, though, from our entire ignorance of the structure 

 of Placohranchus, it is impossible to form a decided opinion as 

 to the identity of the two families. 



Having thus established four families among the Mollusca 

 Nudibranchiata, the next question which suggests itself is, whether 

 these families, when arranged in strict zoological co-ordination, 

 are separated from one another by equal intervals ? The answer 

 must here be at once given in the negative, the Dorididcs being 

 much more nearly allied to the Tritoniadce than these are to any 

 of the remaining families of the order. This circumstance there-^*' 

 fore demands the division of the entire order into two great pri- 

 mary sections, by which means a natural grouping of the families 

 themselves may be effected, and their true relations to each other 

 be rendered apparent. 



The grounds upon which this primary division of the Nudi- 

 branchiata is based, will be found in the singular system of 

 hepatic ramifications, to which attention has already been so 

 frequently directed, and which, though far from being of that 

 importance with which it has been invested by M. de Quatrefages, 

 is yet a decided indication of the existence of two subordinate 

 groups in the order Nudibranchiata. 



