576 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Mollusca. 
Revision of British Mollusca.* — Canon Norman continues his 
revision, and now deals with the Gastropoda ; dealing first with the 
Pteropoda he enumerates six species. The Opisthobranchiata and the 
Nudibranchiata are next enumerated, one hundred and fifty- three species 
being recognized. 
Sensory Organs of Lateral Line and Nervous System of Mollusca.t 
— Dr. B. Rawitz calls attention to certain points in Herr Thiele’s memoir, J 
in which he thinks he has been misrepresented ; and promises to show 
that some of that author’s results are not in correspondence with the 
facts of the case. 
a. Cephalopoda. 
Genesis of the Arietidse.§ — Mr. A. S. Hyatt’s prolonged researches 
on the Arietidse, now published in a large monograph, include an 
account of the genealogy of the three great stocks — Psiloceras, Plicatus, 
and Levis, and their subordinate series ; of the genesis of characteristics 
— progressive, retrogressive, and differential ; of the geological and 
faunal relations; and of the genera and species. The author’s theo- 
retical conclusions are tersely summed up in a preface, and expounded 
in an introductory chapter, but a complete summary would involve an 
explanation of terms exceeding the limits of our space. 
“ Specialization has in all cases appeared to us to be due, not to natural 
selection, but to 'physical selection, or the production of suitable modifi- 
cations by the action of forces which changed in a similar way large 
numbers of the same species, perhaps nearly all the individuals in the 
same locality or same habitat, within a comparatively limited period of 
time.” “We do not intend to dispute entirely the action of natural 
selection and the influence of the struggle for existence, but simply to 
deny the applicability of the law to the more important modifications 
and series of modifications which have occurred in the history of 
animals, taking the fossil Cephalopods as a type.” “ Changes in the 
surroundings acted upon the plastic organism, inducing it to make 
efforts to accommodate itself to new conditions.” “ In so far as causes 
and habits are similar, they probably produce representation or morpho- 
logical equivalence between different series or forms of the same type 
in the same habitat, and in so far as they are different, they probably 
produce the differentials which distinguish series and groups from each 
other.” 
y. Gastropoda. 
Cladohepatic Nudibranchs.il — Prof. R. Bergh emphasizes the con- 
trast between the Steganobranchiata (Tectibranchiata) and the Nudi- 
branchs, but finds connecting links in the order Ascoglossa. The latter 
are allied especially to the cladohepatic Nudibranchs, the holohepatic 
* Ann. and Mag. Nat. Hist., vi. (1890) pp. 60-91. 
f Zool. Anzeig., xiii. (1890) pp. 361-4. % See this Journal, ante, p. 160. 
§ Smithsonian Contributions, xxvi. (1890) 238 pp., 14 pis. and 35 figs. 
i| Zool. Jahrb., v. (1890) pp. 1-75. 
