392 MOLLUSCA. 
diameter is really 16 millim., as stated by Morelet, not 26 millim. as given by Clessin ?, 
whose figures, however, measure not more than 90-21 millim. The form of the 
aperture in Clessin’s figure, moreover, does not agree satisfactorily with that of 
Morelet’s specimen; it is therefore somewhat doubtful whether Clessin had a specimen 
of the same species before him. 
Subgen. GYRAULUS. 
Gyraulus, Agassiz, in Charpentier’s Moll. Terr. et Fluv. Suisse (Mém. Soc. Helv. de Sci. Nat. i.), 
p. 21 (1837); Moquin-Tandon, Moll. Terr. et Fluv. Fr. p. 488 (1855); Hartmann, 
Gasteropoden d. Schweiz, p. 89 (1844) ; Binney, Land and Freshw. Moll. of N. Am. i. 
p. 128. 
Nautilina (J. E. F. Stein, 1850), H. & A. Adams, Gen. Moll. ii. p. 263. 
Shells of small size, with 8-5 somewhat rapidly increasing whorls, rounded or 
obtusely angulated at the periphery. Surface rather dull, not very shining, with faint 
strie of growth, in some species crossed by very faint spiral lines (P. albus, L., from 
Europe ; P. retusus, Morelet, from Central America). Aperture nearly as high as 
broad, usually very oblique. 
Cosmopolitan. 
8. Planorbis maya. (Tab. XXI. fig. 13.) 
Planorbis maya, Morelet, Test. Noviss. i. p. 16 (1849) *; Fisch. & Crosse, Miss. Scient. Mex., 
Mollusca, ii. p. 72, t. 88. figg. 4, 4a, 6, c*; ?Clessin, in Martini & Chemnitz, Syst. 
Conch.-Cab. ed. 2, Limnziden, p. 208, t. 32. fig. 2°; Pilsbry, Proc. Acad. Phil. 1891, 
p. 322%. 
Near P. orbiculus, Morel., but smaller and subangulate. 
Hab. S.E. Mexico: Isthmus of Tehuantepec (Sumichrast ?). 
Yucatan: Merida (Heilprin*+); Tabi (Godman); city of Campeche, in cisterns 
(Morelet }-°). 
W. Guatemala: Antigua, in wells (StoJ/). 
In general form this species approaches the subgenus Jenetus, but it has not more 
than five whorls. Some of Mr. Godman’s specimens, from Tabi, show several white 
radial markings in the first-formed whorls (see fig. 13), which seem to indicate that 
in the young shell there were periodical thickenings in its substance, somewhat as in 
the subgenus Segmentina ; neither Morelet nor Fischer and Crosse mention them. 
It. is rather doubtful whether Clessin’s description and figure really apply to this 
species, as his shell is of larger size (diam. 12, height 3 millim.; that of Morelet and 
Fischer and Crosse measuring diam. 9, height 24 millim.), and has nevertheless 
only four whorls instead of five; but in counting the whorls authors may easil 
disagree. ‘ 
