[Reprinted from SciENCE Proaress, Vol. V., March, 1896.| 
Pie MOKPHOLOGY OF Tie MOELUSEA 
>: ea recent publication of a number of new manuals and 
monographs dealing with the Mollusca offers a favour- 
able opportunity for a review of our knowledge of this 
great phylum of the animal kingdom. It is not fifteen 
years since Professor Lankester’s classical article on Mollusca 
was published in the Eucyclopedia Britannica, yet the con- 
tributions to Molluscan morphology since that date have 
been not only numerous, but in many cases of prime im- 
portance. 
The older method of inquiry, that of the comparison of 
types more or less arbitrarily selected from different groups, 
has been succeeded by investigations more directly in- 
fluenced by the idea of evolution. The comparison of types 
has been replaced by the study of groups. The founda- 
tions of the morphological edifice were laid upon the former 
method ; the superstructure and details are the result of 
the latter. Homologies having been to a large extent 
determined, we now seek phylogenies. It happens also 
from time to time that the detailed study of a group with 
the object of reconstructing the phylogeny of its members 
leads occasionally to the discovery that homologies based 
on the simple method of anatomical comparison turn out 
to be nothing more than analogies—recurrent examples of 
similar modifications. 
One result of these phylogenetic inquiries has been the 
concentration of particular attention upon forms which are 
presumably the most primitive in each group; and great 
advances have thus been made in our knowledge. Kow- 
alewsky and Marion, Pruvot, Wirén, and Thiele have 
enormously extended our acquaintance with the Apla- 
cophorous I[sopleura; primitive Prosobranchs (Docoglossa 
and Rhipidoglossa) have been thoroughly investigated by 
