160 Prof. G. J. Allman on the Anatomy of Actzon. 
described it (Mém. de la Soc. d’Hist. Nat.) as a Nucleobranch 
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 mollusk 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 Mollusca, 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, Doridide, Tritoniade and Eolidide. 'They make more- 
over a most important revision of the old genus Zrztonia, separa- 
ting from it the Tritonia arborescens of previous authors, which 
they find, notwithstanding its divided branchize and general Tri- 
tonia-like appearance, to possess a true Kolidian structure, and 
which they accordingly locate in the family of the Holidide as a 
distinct genus under the name of Dendronotus. 
In order however that Actgon may also find a place among the 
Mollusca 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 Rang, though, from our entire ignorance of the structure 
of Placobranchus, it is umpossible 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 Doridide bemg 
much more nearly allied to the Tritoniade 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. 
