108 The MoUuscan Family Planorhidae 



lacking internal barriers or lamellae in the aperture. The shell of com- 

 planatus, however, is lenticular with a sharply angular peripheral keel and 

 the aperture is longitudinally triangular, wider than high. Helicorbis, 

 on the contrary, has a higher shell with rounded or subangular whorls ancl 

 the aperture is oblique and higher than wide. 



The published description of the anatomy of this group (the type, 

 Planorbis umbilicalis) is insufficient for the purpose of making comparisons 

 with Hippeutis complanatus, as no mention is made of flagella, penial 

 gland, or prostate. The penis (verge) is said to have its sperm canal open- 

 ing on the side, which is like that of Hippeutis (see plate 2, figs. 5, 6) . Until 

 more is known of the genitalia of Planorbis umbilicalis, the group Heli- 

 corbis had better be considered a separate genus related to Hippeutis. 



The name Helicorbis has been almost unnoticed since its publication in 

 1855. Dall (1905) overlooked it in his Alaska Mollusks but later (1906) 

 he considered it equal to Hippeutis Agassiz, 1837. Germain (1921) omits 

 all reference to it. Kennard and Woodward make it a synonym of 

 Hippeutis (1926), as does Thiele (1931). Bartsch, in a paper on Philippine 

 mollusks (1907, p. 84), places a related species in Helicorbis, the latter 

 treated as a subgenus of Planorbis. Bequaert and Clench in a later paper on 

 Philippine Lymnaeidae and Planorbidac (1939, p. 16) place Planorbis 

 umbilicalis in Hippeutis, which is recognized as a genus. 



In Benson's paper (1855, p. 126) three species are mentioned under 

 Helicorbis; hemisphaerula and nitida are ruled out by the remark in the 

 paragraph above concerning Secjmentina (of which nitida is the type) 

 which refers to the internal divisions or barriers. Hemisphaerula, also, is 

 the type of Pilsbry's Polijpylis. There remains the species umbilicalis 

 which has not previously been used as a type for any group and this is here 

 nominated for the type of Helicorbis Benson. 



Helicorbis may bear something of the same relationship to Hippeutis 

 that Polypylis bears to Segmentina, although the anatomical differences 

 between the last two groups are greater than appears to be the case of the 

 first two groups. 



Genus TROCHORBIS Benson, 1855 



Type by original de.signation Trochorbis trochoideus Benson 



1855. Trochorbis Benson, Jour. Asiatic Soc. Bengal, XXIV, p. 126. Type Trochorbis 



trochoideu.s (Benson). As subgenus of Planorbis. 

 1906. Trochorbis Dall. Nautilus, XIX, p. 105. Type Planorbis trochoideus Benson. 



As subgenus of Planorbis. 

 1927. Trochorbis Kenn.ard and Woodward, Syn. Brit. Non-Marine Moll., p. 90. No 



type cited. Considered a synonym of Segmentina. 



Description of the type of the genus Planorbis trochoideus Benson: 

 'Testa vix perforata, diaphana, nitida, inconspicue radiato-striata, subtro- 

 chiformi, convexa; spira parvula, arete convoluta, concava; sutura pro- 

 funde impressa; anfractibus 3i/{>, penultimo valde convexo, ultimo majori, 

 versus suturam obtuse angulato, extus depresso, interne acute carinato, 

 intus laminis rarissimis jileurumque minuto, subtus piano, medio, versus 

 umbilicum inconspicuum, leviter excavato, versus periphaeriam vix declivi; 

 apertura oblic|ua subsagittata, marginc suj^criori arcuato, jn'ominente, 

 inferiori recidente. Diam. 3 mm., axis vel 2' (Benson, 1850, p. 352). 



'The infrequency and irregularity of the internal laminae in this species, 

 causing them to look more like accidental thickenings of the shell, made 



