PTEEOPODA. 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, CI. 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 ( CI. longicaudata, CI. flavescens, and CI. punctata) have not 

 been described in great detail, but they do not seem to difier 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 buccal 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 L. antarctica 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 S. L. helicoides, which is known only by the shell, 

 resembles L. helici7ia 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 diff"er as to whether all 

 the other species are really valid. The relationships of L. helicina and L. antarctica, 

 as well as of L. retroversa, L. lesueuri and L. australis are discussed below. 



There is some difierence of statement as to the presence or absence in this 

 genus of organs called jaws, and possibly some variation in the texture of the 



* SudpolariExpedition. Pteropoden, p. 105, 1906. 



