PLANORBIS. 381 
Subgen. HeEtisoma. 
Helisoma, Swainson, Treatise on Malacology, p. 337 (1840) ; Fischer & Crosse, Miss. Scient. Mex., 
Mollusca, ii. p. 59. 
Planorbis, s. strict., Planorbella, and Helisoma, Binney, Land and Freshw. Shells N. Am. ii. 
pp. 103, 109, 112. 
Shells of comparatively large size; whorls 3-5, swollen, rapidly increasing, obtusely 
rounded at the periphery, but usually with a more or less distinct spiral angular line 
near the left, and also in some species near the right, suture; sculpture consisting of 
broad striz of growth; surface not very shining, pale yellowish. Aperture usually 
higher (parallel to the axis of the spiral volution) than broad (in the largest diameter 
of the shell), or at least as high as broad, often very asymmetrically triangular. 
This subgenus is chiefly confined to the continent of North America, but it is also 
represented in South America by P. peruvianus, and in the East Indies by P. indicus, 
Desh. (coromandelicus, Beck), and the well-known European P. corneus, L., comes 
rather near it. 
1. Planorbis tenuis. (Tab. XXI. fige. 1; 7, 7a, young.) 
Planorbis tenuis, Dunker, in Martini & Chemnitz, Syst. Conch.-Cab. ed. 2, Limneacea, p. 45, t. 9. 
figg. 14-19, and t. 16. figg. 22-25 (1850*)1; v. Mart. Malak. Blitt. xii. p. 55 (1865) *; 
Binney, Land and Freshw. Shells N. Am. ii. p. 118, fig. 189 (copied from Dunker) *; 
Strebel, Beitr. Mex. Land- und Siissw.-Conch. i. p. 42, t. 5. figg. 21-26, a, 6, c*; Fisch. & 
Crosse, Miss. Scient. Mex., Mollusca, ii. p. 60, t. 34. fig. 1°; Pilsbry, Proc. Acad. Phil. 
1891, p. 821°. 
Planorbis mexicanus, Ziegler, in litt.". 
Planorbis fragilis, Dunker, in Martini & Chemnitz, loc. cit. p. 46, t. 10. figg. 41-43 (young shell) °; 
Binney, loc. cit. p. 122, fig. 203 (copied from Dunker) °. 
Allied to the North-American P. trivolvis, Say: right side deeply hollowed in the middle, with very convex, 
rapidly increasing whorls; on the left side all whorls, except the last, forming a plane level overtopped by 
the last and occupying no more than 4 of the greatest diameter of the shell. Aperture somewhat trian- 
gular, the left margin angularly bent near the insertion and then ascending very obliquely to the plane of 
the spiral. ; 
a. Diam. maj. 22, min. 17, alt. 114; apert. diam. 103, ampl. 113 millim. 
b. 9 184, ,, 13, , 9; ” 84, 4, 92 4 
a. From Dunker’s figure; 6. From a specimen in the Berlin Museum. 
Hab. N. Centra Mexico: Durango (Seemann, in coll. Dunker, given by H. Cuming 
as P. twmens). 
CrentraL Mexico: ditches in the neighbourhood of the city of Mexico, plentifully, in 
company with Limnea attenuata (Schiede & David1*%; Deppe & Uhde?*; 
Heilprin®) ; Mexico city (Major Rich *) ; Lake of Chalco (Méhédin® ; Heilprin ®) ; 
* Pp, 21-62 of this work were published in 1850, according to the annual records in Troschel’s ‘ Archiv 
fiir Naturgeschichte,’ xvii. 2, pp. 101, 129. 
