390 MOLLUSCA. 
proportional relation of the height of the shell to its diameter, which is greater in 
P. tenuis, but individually variable in both. P. caribeus bears a similar relationship 
to P. tenuis that P. glabratus, Say, does to P. trivolvis, or, within Europe, P. banaticus 
to P. corneus. Amongst the species of the subgenus Helisoma, P. caribeus approaches 
nearest the subgenus Menetus. 
Haldeman (Monogr. Limniades, vii. p. 12) says that figg. 1 and 3 of the tab. 2 of 
his P. glabratus were drawn from Mexican (?) shells; they somewhat resemble 
P. caribeus. 
Subgen. MENETUS. 
Menetus, A. & H. Adams, Gen. Moll. ii. p. 262 (1858). 
Shells of moderate size and rather depressed form ; whorls more than five, gradually 
or more rapidly increasing, rounded or slightly angulated at the periphery. Surface 
smooth. Aperture as broad as or broader than high; dextral and sinistral margins 
not much arcuated nor angular. 
This subgenus is mainly peculiar to South America (P. cumingianus, Dunk.) and 
the Antilles (P. guadeloupensis, Sow.); the species inhabiting Mexico and Central 
America are chiefly recognizable by the large number of whorls. 
It is distinguished from Helisoma by various characters, though there are several 
intermediate forms, i. e. P. caribeus, from Mexico, P. belizensis, from Central America, 
and P. bahiensis, from Brazil. 
5. Planorbis belizensis. 
Planorbis belizensis, Crosse & Fisch. Journ. de Conch. xxvii. p. 342 (1879) *; Miss. Scient. Mex., 
Mollusca, ii. p. 68, t. 32. figg. 6, 6a, 6°. 
Hab. 8.E. Mexico: Tabasco (Morelet ). 
British Honpuras: Belize (Bocourt 1). 
N. Guaremata: Coban and San Miguel Tucuru (Bocourt 2). 
This species seems to form a connecting-link between the subgenera Helisoma and 
Menetus: the aperture is scarcely oblique, and the left margin of it ascends rather 
obliquely outwards, as in Helisoma; but the number of whorls (6) and the general 
appearance bring it near the following, P. orbiculus. 
6. Planorbis orbiculus. 
Planorbis simplex, Beck, Index Moll. p. 120 (1837) (sine descr.) '. 
Planorbis orbiculus, Morelet, Test. Noviss. i. p. 17 (1849) °; Reeve, Conch. Icon. xx., Planorbis, 
t. 8. fig. 20°; Fisch. & Crosse, Miss. Scient. Mex., Mollusca, ii. p. 70, t. 82. fige. 9, 9a,b,c*; 
Clessin, in Martini & Chemnitz, Syst. Conch.-Cab. ed. 2, Limneiden, p. 188, t. 17. fig. 12°; 
Pilsbry, Proc. Acad. Phil. 1891, p. 822°. 
