ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
615 
fication followed is practically the same as that used by Mr. Hoyle in 
his * Challenger’ Report. Twenty-four genera with thirty-eight species 
are dealt with in the monograph. 
y. Gastropoda. 
Revision of North American Slugs.* — Messrs. H. A. Pilsbry and 
E. G. Yanatta have made a revision of the genera Ariolimax and Aphal- 
larion. They remark that the genera of slugs inhabiting North America 
have hitherto been diagnosed by external characters, and by those of the 
jaw and teeth. They purpose to indicate in a series of papers, of which 
this is the first, some of the more important of the internal features, 
particularly the genitalia and alimentary canal. With regard to the 
genera selected, we are told that no correct figures or descriptions of 
their genitalia have yet been published. The authors now, for the first 
time, make known the true structure of the male organs of Ariolimax . 
Aphallarion is proposed for a new, perhaps the largest American slug, 
which is remarkable for lacking a penis. The two genera selected 
appear to be both closely allied to Avion , but this last, of course, has 
only been introduced into the American fauna. The details of this 
paper will be of interest and importance to those who are specially 
engaged in the study of slugs. 
Insular Land Shell-Faunae.f — Mr. W. H. Dali has an interesting 
essay on insular land shell-faunae, especially as illustrated by the data 
obtained by Dr. G. Baur, in the Galapagos Islands. After some account 
of the general characteristics of the islands of the archipelago, Mr. Dali 
reports that twenty-seven forms which can be defined from one another 
were collected from seven out of the twelve principal islands by Dr. 
Baur. The results of this naturalist leave little room for doubt that a 
thorough examination of all the islands would add materially to the 
number of distinct forms, and therefore that the time for finally discuss- 
ing or speculating upon the distribution of the species among the 
several islands has not arrived. Nearly all the land-shells of the Gala- 
pagos are more or less arboreal, and pass much, if not the whole of the 
dry season attached to branches of trees by a deposit of tough dry 
mucus forming a hermetic seal to the aperture as well as a means of 
fixation. The distinctively American type of the land shell-fauna 
indicates its point of origin. The author summarises his general results 
in the following way : — 
(a) Given a region of volcanic origin and mountainous character, 
with local or seasonal aridity, more or less arboreal vegetation as well as 
herbage, and a tropical or nearly tropical climate, moderate isolation, 
and safety to propagate and increase. 
(b) Into this region let land shells of the principal continental types 
be introduced, and allowed the necessary time to become dispersed over 
the region, multiply abundantly, and respond to the environment. 
(c) What results in the shape of dynamic modifications may be 
anticipated? The first result of room to spread, safety, and plenty of 
food, would be to release the species from the shackles of the environ- 
ment from which they have been transplanted, and to promote variability. 
* Proc. Acad. Nat. Sci. Pliilad., 1896, pp. 339-50 (3 pis.), 
f Tom. cit., pp. 395-459 (3 pis.). 
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