36 



NATURAL SCIENCE. 



July, 



that organ. The liver in Prosobranchs undergoes a process of 

 evolution comparable to that of the other organs ; but the sub- 

 divisions that could be established for the Pectinibranchiata on its 

 variations would not entirely correspond with those founded on other 

 characters, such as the radula, nervous system, kidney, or shell. 

 Hence the difficulty of founding a classification on the sum of the 

 characters, and the desirability — M. Fischer considers, since the liver 

 is not suitable for the purpose — of basing a systematic arrangement 

 on some single feature, such, for example, as the radula. 



The classification of the Mollusca, especially from a phylogenetic 

 aspect, has received of late very much attention, and quite a forest of 

 genealogical trees have been constructed for this sub-kingdom, or 

 portions thereof, by Von Jhering and others, to the great bewilderment 

 of the unlucky student who attempts to thread their maze. 



The most recent complete scheme is probably (there may be a 

 later by the time this appears in print) that by Dr. P. Pelseneer (6), 

 and i"t is a matter for regret, that while the privileged few have had 

 author's copies for some months past, so that his table has been even 

 incorporated in Simroth's new edition of the molluscan portion of 

 Bronn's great work (7), the volume from which it was supposed to be 

 taken has but just been made available for the many. 



On looking at the scheme here reproduced in both its tabular 

 and phylogenetic form (the latter modified in accordance with a later 

 paper to be presently mentioned), certain points at once engage the 

 attention : most of them, it is true, have been already set forth in 

 minor communications, but the arrangement, as a whole, has not 

 been presented before, and therefore it appears a fitting opportunity 

 on which to call attention to them : — 



Amphineura 



Gastropoda 



Scaphopoda 



Lamellibranchia 



Cephalopoda 



f Polyplacophora 

 \ Aplacophora 



Streptoneura 



Euthyneura 



I Aspidobranchia 

 I Ctenobranchia 



I Opisthobranchia 

 I Pulmonata 



Solenoconcha 



f Protobranchia 

 Filibranchia 

 . - Pseudolamellibranchia 

 Eulamellibranchia 

 [_ Septibranchia 



( Dibranchia 

 ( Tetrabranchia 



I Rhipidoglossa 

 ( Docoglossa 



I Platypoda 

 I Heteropoda 



I Tectibranchia 

 (Nudibranchia 



I Stylommatophora 

 I Basommatophora 



In the first place, the Pteropoda as a class have disappeared, 

 being merged with the Opisthobranchiata. They were originally 

 classed as allies of this last-named order by De Blainville (8 and 9), 

 who then used the term " Pteropoda " for certain of the Nucleobranchs 



