78 The Molluscan Family Plauorbidae 



Geographical Distribution. Armiger is Palearctic in distribution, oc- 

 curring in the northern United States and Canada, northern Europe and 

 Asia. Armiger crista is reported from the trans-Mediterranean region, 

 from Algeria, Tunis, etc. A species {Armiger annandalei Germain) has been 

 described from eastern Asia. The genus contains several forms which 

 should probably rank as species. 



Geological Distribution. According to Wenz (pp. 1632-1633) Armiger 

 is first known from the Middle Miocene period. 



Species Considered as Valid. The only species definitely assigned to 

 this genus in the present volume are: 



Armiger crista (Linn.) Armiger imhricatus (Miiller) 



Armiger crista spinulosum Armiger annandalei (Germain) 



(Clejsin) 



Remarks. By the form of the penial complex tliis group is distinct from 

 Gyraulns and should be considered as of full generic rank. L. Soos (1917) 

 and A. Soos (1935) list it as a subgenus of Gyraulus, but the lack of a 

 penial stylet removes it from that group. Lindholm (1926) considers it a 

 genus. Thiele (1931) considers it a subgenus of Anisus. Odhner (1929, 

 p. 30) gives the group full generic rank on the basis of the lack of a stylet 

 on the verge which is characteristic of Gijraulu^^. A good figure of the verge 

 of Armiger crista, showing the small penial jjapilla at one side of the sperm 

 outlet, is shown (fig. 10). Haas (1929, p. 386) considers Armiger a distinct 

 genus. 



Odhner (1929, pp. 20, 21, 30) states his belief that Armiger represents 

 a stage in the evolution of Gyraulus. He found young Gyraulus borealis of 

 two whorls without a stylet on the verge while those of three and one-half 

 whorls had a well-developed stylet. Specimens of Gyraulus laevis of three 

 whorls had a stylet. From these observations he concludes that the develop- 

 ment of the stylet (penisstachel) may be checked in the later fall, possibly 

 through some unfavorable climatic condition. Haas, in a review of this work 

 ( 1930, p. 20) does not fully agree with Odhner, believing that more study 

 should be given the subject before conclusions are made. This is undoubt- 

 edly true, but the subject is one of great interest and dissections should be 

 made of many species at different ages to determine whether the stylet is 

 absent in young and immature Gyraulus. It has been present in all speci- 

 mens personally examined, but these were either full grown or nearly 

 full grown. 



This group of small snails is highly variable specifically and a number 

 of names have been bestowed upon the variations. Two forms appear dis- 

 tinct enough to rank as species, crista (Linn.), with strong ribs projecting 

 from the shell, and imbricatus (Miiller), in which no ribs project from the 

 shell. Armiger spinulosum (Clessin) is i^robably a race of crista Linn. 

 Another species in eastern Asia has been named annandalei by Germain. 



The value of recognizing subgenera for these variations may be ques- 

 tioned seriously. In 1922, Dybowski and Grochmalachi erected three sub- 

 genera based on spiny variation in Armiger: Nautiloarmiger, Cristoarmi- 

 ger, and Atropoarmiger (vide Lindholm, 1926, p. 24). 



There appears to be some variation in the genitalia and also in the 

 radula. Odhner found the formula to be 11-1-11 and 14-1-14 in crista from 

 two localities and the writer obtained the formula 16-1-16 from crista 



