156 



PKOCEEDINGS OF THE MALACOLOGICAL SOCIKTY. 



obvious tliat Miiller regarded the smaller Planorbidoe, which formed 

 the bulk of his genus, as the typical members, and utilized the 

 suggestive name of an old well-established species of this group, first 

 described some seventy years before when the binomial nomenclature 

 had not yet come into use, to designate it by. In other words 

 H. planorhis is the type of Planorhis by tautonomy, and the efforts of 

 Swainson or any other writer fifty or more years later to transfer the 

 name to a different group should not be tolerated. If lack of classical 

 examples is claimed, it might be noted that in 1837 Charpentier, 

 utilizing some of Agassiz's manuscript work, first subdivided the 

 genus as it now stands into natural groups, correctly restricting 

 Planorhis to the section including H. planorhis, L., and II. vortex, L. 



The genus Planorhis in the sense used here embraces the so-called 

 smaller Planorbes, and is characterized particularly by the peripheral 

 keel in typical Planorhis, the simple rounded succeeding stage the 

 denticulations developed in the ' throat' of the shell in the sub-genus 

 SegmeJitina, and a second ' round-whorled ' stage following that. 

 These denticules take the form of plaits or cusps, and differ radically 

 in each species, thus affording a ready means of identification, and 

 incidentally several unnecessarj' sectional names. P. nitida, of Europe, 

 has a transverse columellar plait and two on the whorl. The American 

 P. armigera is more complex, possessing cusps in addition to the plaits, 

 wliich are in tliis instance diagonal. The appearance on looking into 

 the aperture when the shell is oriented in its natural position might be 

 diagrammatically expressed as follows : — 



P. nitida. P. armigera. 



> 

 \ 



> 



Planorbis (Gyraulus) albus (Miiller). 

 Planorhis alhus, Miiller, 1774; P. deflectus, Say, 1824; P. hirsutus, 

 C. B. Adams, 1839; P. vermiciilaris, Gould, 1847 ; P. horealis, 

 ' Loven MS.,' Westerlund, 1875. 

 Shell small, whorls rounded, fairly deep, and usually more or less 

 hirsute, spire-pit narrow for group ; habitat chiefly in lakes, preferring 

 deep water. 



Eoreal portions of Paloeai-ctic and Nearctic Regions. Yukon, Alaska, 

 Fraser, Columbia, Klamath, and Coast Range (locally) Systems. 

 Quaternary : Loess of eastern States. 



Planorbis (Gyraulus) parvus (Say). 



Planorhis parms. Say, 1817; P. glaber, JcSveys, 1820 ; P.levis, Alder, 

 1838 ; P. ehvatus, C. 13. Adams, 1840; P. vermicularis of authors 

 in part, not of Gould. 

 Shell small, whorls compressed, spire-pit widely evenly concave ; 

 habitat lakes and streams. 



