Prof. G. J. Allman on the Anatomy of Acteon. 159 
in the united branchiz of an Kolidian mollusk ; and if to this we 
add the gastric or hepatic ramifications, and consider the general 
character of the anatomy as detailed m the present paper, we can 
have no hesitation in making Act@on a genus of NUDIBRANCHIATE 
Mo.uvusca. As to the close affinity of Acteon 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 Placobranchus, 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 Polybranchiata and 
Cyclobranchiata, the former corresponding to the family T7ito- 
niade of subsequent zoologists, and the latter to that of Dori- 
dide. De Blamville divides the Polybranchiata into two minor 
groups, Tetracerata and Dicerata, both natural, the former in- 
cluding Holis, Glaucus, &c., and the latter Tritonia, Scyllea and 
Thetis. 
With the position here assigned to Tritonia, Scyllea and The- 
tis, though the group is in itself natural, | cannot concur, as I 
believe these mollusca much further removed from Holts 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 Ptérosomes, 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, Blamv.; 
4. les Doris = Cyclobranchiata, Blainy. ; 5. les Placobranches, esta- 
blished for the Placobranchus of Van Hasselt. 
Pterosoma, wpon 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, 
