KENNARD : ON PLAN0RBI8 VOETIGULUS AND P. LJEVIS. 47 



a white colour, more polished and transparent, and is destitute of any 

 markings. The upper side is uniformly more even, and the under 

 exceedingly concave." It will at once be noticed that this description 

 does not quite agree with P. Icevis, for that species is never found 

 with P. atbiis, and it is not white. 



Moreover, examples of P. glaher sent by Jeffreys to Alder are stated 

 by the latter to be only a variety of P. alius, ^ and it was considered as 

 such by J. E. Gray and Foi'bes & Hanley. That P. glaher, Jeif., of 

 "British Conch ology " is P. Icevis, Alder, there can be no doubt, but 

 it appears to me that the P. glaher, Jeff., of the Linn. Trans, is only 

 a variety of P. albus. 



Under these circumstances and in justice to the memory of one of 

 our most competent malacologists, it is advisable to relegate glaher, 

 Jeff., to the synonymy of alhus, Miill., and to revive the use of Icevis, 

 Alder. 



The question, however, whether we should use parvus. Say, is 

 another matter. 



There are in the Holarctic region several described species which 

 conchologically have considerable resemblance to each other. These 

 include lavis. Alder, parvus. Say, arcticus, Beck, Sihiricus, Dunker, 

 Rondeli, Bourg., vermicularis, Gould, and Rossmaesslern, Auers. 

 Practically nothing is known of the anatomy of these forms, and it is 

 quite impossible to state whether these are geographical races of one 

 species, or whether we are dealing with genuine species with 

 a superficial likeness. In any case, it is better to retain the specific 

 name Icevis, for though similar to the American parvus it is not 

 identical. 



It is, I think, worthy of note that both the Palsearctic and 

 Nearctic regions possess a group of three species in the Gyraulus 

 group. In North America we have P. hirsutus, Gould, P. deflectus. 

 Say, and P. parvus, Say, and in Europe there are P. alhus, Miill., 

 P. Stroemii, West., and P. Icevis, Alder, and there is great similarity 

 between the species in the order given. This may be a mere 

 coincidence, but it is possible that it may be a phenomenon in the 

 evolution of species. 



3. On some proposed Subdivisions of the Genus. 



Dr. W, H. Dall has recently elaborated a new scheme of classi- 

 fication for Planorhis with some new subdivisions,^ and there are two 

 proposed new sections which, in my opinion, are open to criticism. 

 Dr. Dall's divisions are as follows^ : — 



Section Diplodiscus, "Westerlund (restricted). 



Shell small, with numerous slowly enlarging whorls, keeled or 

 angulate from the beginning. Type, P. vortex, Linn. 



1 Mag. Zool. Bot., vol. ii (1837), pp. 113-14. 



2 Dr. W. H. Dall, Land and Freshwater Mollusca of Alaska ; New York, 1905. 

 ^ Op. cit., pp. 85-6. 



