Prof. G. J. AUman on the Anatomy of Actseon. 159 



in the united branchise of an Eolidian mollusk ; and if to this we 

 add the gastric or hepatic ramifications, and consider the general 

 character of the anatomy as detailed in the present paper, we can 

 have no hesitation in making^c^^ow a genus of Nudibranchiate 

 Mollusc A. As to the close affinity of Actceon to Placobranchus, 

 I fully agree in the opinion of M. Sander Rang, expressed in the 

 note just alluded to. Indeed I believe the relation between these 

 mollusca to be closer than has been yet suspected, though, from 

 the imperfect state of our knowledge of PlacobranchuSj it would 

 be at present premature to urge with confidence any further opi- 

 nion upon this subject. 



While I have thus strongly objected to the establishment of a 

 new order for the reception of the Eolidian Nudibranchs, I yet 

 believe that strict zoology peremptorily demands the formation 

 among the Nudibranchs of a distinct group for these mollusca, 

 by which they may be kept apart from other Nudibranchs with 

 which many zoological writers have too closely united them. In- 

 deed the light which has of late years been thrown upon the ana- 

 tomy of the Mollusca Nudibranchiata places us in a position for 

 recognising those relations by which a natural' subordinate group- 

 ing of the order may be effected. A dismemberment founded 

 upon the differences of organization of the Mollusca Nudi- 

 branchiata had been to a certain extent carried out by M. de 

 Blainville in the establishment of his groups Poh/branchiata and 

 Cyclohranchiata, the former corresponding to the family Trite- 

 niadcB of subsequent zoologists, and the latter to that of Dori- 

 didcB. De Blainville divides the Polybranchiata into two minor 

 groups, Tetracerata and Dicerata, both natural, the former in- 

 cluding Eolis, Glaucus, &c., and the latter Tritonia, Scyllcea and 

 Thetis. 



With the position heye assigned to Tritonia, Scyllcea and The- 

 tis, though the group is in itself natural, I cannot concur, as I 

 believe these mollusca much further removed from Eolis and its 

 allies than from Doris. 



Sander Rang (Man. des Mol.) rejects De Blainville^s groups 

 Polybranchiata and Cyclobranchiata, and primarily divides the 

 entire order into five families : 1 . les Pterosomes, established for 

 the reception of a single genus Pterosoma, discovered by Lesson 

 in the equatorial seas ; 2. les Glauques = Polybranchiata Tetra- 

 cerata, Blainv. ; 3 . les Tritonies = Polybranchiata Dicerata, Blainv. f 

 4. \q^ Doris =^Cyclobranchiata,^\ii\n.w.', 5. les Placobranches, ests.- 

 blished for the Placobranchus of Van Hasselt. 



Pterosoma, upon which Rang founds his first family, is cer- 

 tainly a very doubtful Nudibranch, and I believe admitted into 

 this order upon very uncertain grounds. Lesson, its discoverer, 



