PTEROPODA. 3 
I confess that I have seen no explanation of these facts which appears to me 
satisfactory. Our knowledge of the direction in past ages of ocean currents which must 
have largely determined the distribution of pelagic forms is slight, and our record of 
fossil Pteropods is very imperfect. As far as I can ascertain, none are recorded from 
South America or South Africa, but it hardly seems possible to argue profitably about 
the distribution of the group in the past without definite information on this point. 
Meanwhile it is interesting to observe that one Antarctic form, Clio sulcata, is 
closely allied to a cosmopolitan form, Cl. pyramidata, and may plausibly be considered 
as a special adaptation of it to Antarctic life. Also, if Limacina lesueuri is admitted to 
be merely a variety of L. retroversa, then L. retroversa is cosmopolitan and bipolar. Is 
it not probable then that Clione antarctica and Clione limacina, plus some tropical 
forms of the genus, represent variations of a once cosmopolitan species? There 
is nothing unnatural in the idea that such a species may have undergone similar but 
not identical changes in North and South Polar waters. The species of Clione 
inhabiting the warmer seas (Cl. longicaudata, Cl. flavescens, and Cl. punctata) have not 
been described in great detail, but they do not seem to differ from the Arctic and 
Antarctic species so profoundly as to forbid the supposition that all may be modifica- 
tions of one form. It is noticeable that the Arctic and Antarctic species have 
invariably three pairs of buceal cones, whereas the warm water species have two pairs 
or only one. The forms of Limacina which predominate in the tropics are not nearly 
allied to ZL. antaretica and L. helicina, but Dr. Meisenheimer states that L. rangi 
“ weist ausserordentlich nahe Beziehungen zu L. helicina auf,” * although he separates 
the two. This species, as to whose independence authors are not agreed, has been 
found as far north as Lat. 33 8. LZ. helicoides, which is known only by the shell, 
resembles L. helicina and L. antarctica in having a flat spire, though it is specifically 
distinguishable. It is widely, though sparsely, distributed in the warmer waters of the 
Atlantic. 
The anatomy of the Pteropods has been so fully described by various authors that 
in the following notes I have not touched on it, except when necessary for purposes of 
classification. My best thanks are due to Mr. T. J. Evans, Lecturer on Zoology in the 
University of Sheffield, for preparing sections and drawings, and for much assistance. 
LIMACINA. 
Ten or eleven species have been referred to this genus, but the animals of 
L. triacantha and L. helicoides are unknown, and opinions differ as to whether all 
the other species are really valid. The relationships of L. helicina and L. antarctica, 
as well as of ZL. retroversa, L. lesueuri and L. australis are discussed below. 
There is some difference of statement as to the presence or absence in this 
genus of organs called jaws, and possibly some variation in the texture of the 
* Siidpolar, Expedition. Pteropoden, p. 105, 1906. 
