REVIEWS. 611 
typal characteristics of the branch, which were admirably defined by Car- 
penter in 1854, and by Dana in 1863, and Mr. Morse in 1865,* and nothing 
is said of the embryology of either the animals or the shells, omissions 
which are unpardonable in an essay on classification. 
The subdivision of the branch into Mollusca and Molluscoida also ap- 
pears to us objectionable. If Prof. Huxley had drawn his dividing line 
between the Lamellibranchs and Pteropods we should have had an emi- 
nently natural division of the Mollusca, but in placing the line between 
the Lamellibranchs and the Ascidians he repeats a common error. 
The Polyzoa, Brachiopoda, Ascidia and Lamellibranchiata € to the 
Pteropoda, Gasteropoda and Cephalopoda, within their own type, a nega- 
tive relation comparable to that which the invertebrata sid to the 
vertebrata; they are, when contrasted with the last three classes, as 
hole, without a special cephalized extremity. ll the three higher 
classes have the cephalic region distinctly differentiated from the mantle, 
or cenccial region, whereas in all the lower classes the organs of this 
region are buried in the d or ceeneecial region, except among the 
Polyzoa, where they are distinct from the cceneecial region, and may be 
extended in the higher genera, but this differentiation is gained only by 
elevating the cephalic organs to the posterior pole of t dy. 
: he close structural affinities of the Polyzoa and Brachiopoda are no- 
the structure of the Polyzoa and the Brachiopoda, whilst the homologies 
existing between the Ascidia and the Polyzoa are of a much more gene: 
character than those existing between the Ascidians and the Lamelli- 
branchs. 
The Polyzoa and Brachiopoda together may be considered as on 
"ed type, and deflned as a sac closed at one end by a disc, sur- 
unded by free tentacles, and bez by an edentulous mouth from 
which hangs the alimentary ca 
Among the Ascidia and Earaettieosaotiate on the other hand the tenta- 
cles, or gills, are always joined by an intermediate membrane, and they 
together with this membrane form either an open or closed pouch per- 
cte at its lower end by the mouth, from which hangs the alimentary 
anal. 
The atrial chamber has but one aperture in an invaginated Polyzoón or 
a Brachiopod, whereas with the Ascidia and Lamellibranchiata there 
are two. The muscular systems of the Brachiopoda and Polyzoa are 
complicated and homologically similar, as shown by several writers, 



5. 
Penter . eJ 
iil wl ios Dana’s Manual of Geology, and Mr. Morse’s 
a: £4 

E Claseidcation Mollusca in the P. g 

