398 MOLLUSCA. 
Hab. SE. Mexico: Balancan, State of Tabasco (Morelet * *). 
N. GuatemaLa: Rio Usumacinta ’. 
E. Guaremata: Lake of Yzabal (Morelet ? * *). 
The shell figured by Sowerby! without doubt belongs to the species described and 
figured by Fischer and Crosse **; it had been previously noticed, but not described, 
under the name “ P. isabel” by H. & A. Adams [Gen. Moll. ii. p. 264 (1858)]. 
Subgen. PLANORBULA. 
Planorbula, Haldeman, Monogr. Limniades, Suppl. to part 1, p. 2 (1840); H. & A. Adams, 
Gen. Moll. ii. p. 265; Binney, Land and Freshw. Shells of N. Am. il. p. 136; Fischer & 
Crosse, Miss. Scient. Mex., Mollusca, i. p. 76. 
Shell convex, not particularly glossy, with oval, very oblique aperture, and a set of 
internal tooth-like thickenings in the interior somewhat behind it. 
Distinct from Segmentina, Flem., by the absence of the particularly glossy surface 
and the disposition of the internal thickenings, which in Planorbula follow the spiral 
direction and are present in one set only, those first formed being absorbed before the 
later ones appear, while in Segmentina the thickenings run across the whorl and are 
present in several sets. The absence of these teeth in some specimens (P. dentiens, 
var. edentula) shows that this character cannot be regarded as of generic value. 
Known from North America, Central America, and the Antillean islands; one 
species in Egypt (P. alerandrinus, Ehrenberg). 
22. Planorbis obstructus. 
Planorbis obstructus, Morelet, Test. Noviss. i. p. 17 (1849) ’. 
Planorbula obstructa, Fisch. & Crosse, Miss. Scient. Mex., Mollusca, ii. p. 278, t. 33. figg. 8, 8 a-d, 
and t. 34. fige. 7, 7a-d*; Pilsbry, Proc. Acad. Phil. 1891, p. 322°. 
Segmentina doubilli, Tristram, P.Z.S. 1861, p. 232 * 
Planorbis doubilli, Fisch. & Crosse, loc. cit. p. 80°. 
Planorbis berendti, Tryon, Amer. Journ. of Conch. ii. p. 10, t. 2. figg. 14-16 (1866) °; Strebel, 
Beitr. Mex. Land- und Siissw.-Conch. i. p. 45, t. 5. fig. 287; Clessin, in Martini & 
Chemnitz, Syst. Conch.-Cab. ed. 2, Limneiden, p. 198, t. 30. figg. 3, 4°. 
Hab. E. Mexico: Quilate, near Misantla (Hoge); Orizaba (Botteri®); Vera Cruz 
(Liebmann, in coll. Dunker ; Hoge); Rio Tenoya, Vera Cruz, and in the alluvial 
deposits on the beach (Strebel ”). 
-S. Centra Mexico: San Carlos, east of Oaxaca (Hoge); Isthmus of Tehuantepec 
(Sumichrast 2). : 
Yucatan: Carmen Island (Morelet1?); Ticul (Hetlprin®). 
W. GuatemaLa: Lake of Duefias (Salvin **); Escuintla (Conradt, in Mus. Berol.). 
W. Panama: Panama (Dunker, in coll. Albers). 
The Vera Cruz specimens in the Berlin Museum vary from 4~54 millim. in 
diameter, 12-2 in height, and 12-2 in diameter of the aperture; but Strebel? gives 
