384 MOLLUSCA. 
Lake Chapala, near Guadalajara, State of Jalisco (Fischer & Crosse*®); Jaral, 
State of Guanajuato (Schumann). 
E. Mexico: Jalapa and Cordova (Hége), Orizaba (Strebel *). 
P. peruvianus, Brod., is, according to the specimens in the Berlin Museum, very near 
this species, but it is more compressed, with more feeble strie, and of a paler colour. 
The young shell of P. tenuis (figg. 7, 7a) is remarkably higher in proportion to its 
diameter than the adult one, as in all species of this genus; it is very well figured by 
Strebel 4 (fig. 21 5). 
The most important forms of this variable species, besides the typical one, are the 
following :— 
Var. boucardi. 
Planorbis tenuis, var. boucardi, Fisch. & Crosse, loc. cit. p. 61, t. 382. figg. 8a, 6"; Pilsbry, 
loc. cit. p. 822”. . 
Whorls increasing more rapidly in width, those on the right side thus appearing to be more deeply excavated, 
two only clearly visible ; left side as in the typical form; aperture very large, rounded. 
Diam. maj. 18, min. 15, alt. 124; apert. diam. 74, ampl. 123 millim. 
Hab. Centrau Mexico: environs of the city of Mexico (Boucard 1°); Lake of Chalco, 
in company with typical specimens (Heilprin »). 
Probably an individual variation. 
Var. juvenilis,n. (Tab. XXI. fig. 4.) 
Planorbis solidus, Wiegmann, in Mus. Berol. (nec Dunker, in Martini and Chemnitz, 1852)”. 
Planorbis tenuis, var., Strebel, loc. cit. pp. 42, 48, t. 5. fig. 21¢™. 
Also comparatively very high, with rapidly increasing whorls ; right side very deep, the left side very shallow ; 
aperture very large, asymmetrically triangular. 
a, Diam. maj. 183, min. 133, alt. 84; apert. diam. 9, ampl. 104 millim. 
b. ” 153, ” 12, ” 84; ” 74, ” 83 ” 
a. Strebel’s specimen ; 6. Specimen in the Berlin Museum, here figured. 
Hab. CextraL Mexico: ditches near the city of Mexico (Deppe}!*); city of Mexico 
(Strebel 33). 
This is in some respects a continuation of the juvenile form into adult age, retaining 
the comparative altitude (breadth) which is characteristic of the young shell. It 
resembles the North-American P. corpulentus, Say, but differs from it in the less 
distinct sculpture and the more asymmetrical aperture. 
Var. applanatus,n. (Tab. XXI. fig. 3.) 
Whorls less rapidly increasing, one-half more in number; right side without spiral angular line on the first 
whorls ; left side more deeply hollowed, with the spiral angle nearly obsolete. Aperture typical, but 
rather small, 
Diam. maj. 24, min. 19, alt. 12; apert. diam. 104, ampl. 12 millim. 
Hab. Cuntrat Mexico: Plateau of Mexico (Uhde). 
