464 MOLLUSCA. 
SEMISINUS. 
Hemisinus, Swainson, Treatise on Malacology, 1840, pp. 200, 341 ; H. & A. Adams, Gen. Moll. i. 
p. 802; Brot, in Martini & Chemnitz, Syst. Conch.-Cab. ed. 2, Melaniaceen, p. 368. 
Semisinus, P. Fischer, Manuel de Conch. p. 701 (1885); Fischer & Crosse, Miss. Scient. Mex., Moll. 
ii. p. 313. 
General shape of Melania and Pachychilus, but with the base of the columellar 
margin notched obliquely and therefore separated distinctly from the basal margin. 
The Central-American species exhibit a strong spiral sculpture. 
Operculum paucispiral, horny, the nucleus near its lower end. 
Edges of the mantle fringed, as in the Old-World Melanie. 
Radula comparatively shorter than in Pachychilus; central tooth subquadrangular, 
multicuspidate, with a slight elevation in the middle of its plane; lateral tooth multi- 
cuspidate ; marginal teeth very long, quadri- or quinque-cuspidate. 
Viviparous. . 
The anatomy of the Central-American species is given by Fischer and Crosse, loc. cit. 
Semisinus is a philological correction of the hybrid word Hemisinus. 
1. Semisinus ruginosus. 
Melania ruginosa, Morelet, Test. Noviss. i. p. 25 (1849) ’. 
Hemisinus ruginosus, H. & A. Adams, Gen. Moll. i. p. 803%; Reeve, Conch. Icon. xii., Melania, t. 3. 
figg. 18, 136°; Brot, in Martini & Chemnitz, Syst. Conch.-Cab. ed. 2, Melaniaceen, p. 394, 
t. 41. fig. 1*, and var. , p. 395, t. 41. fige. 2,2a°; Stoll, Guatemala, Reisen u. Schilder- 
ungen, 1886, p. 461 °. 
Semisinus ruginosus, Fisch. & Crosse, Miss. Scient. Mex., Moll. ii. p. 818, t.50. figg. 11, ll a-d’. 
Melania petenensis, Tristram, P. Z. S. 1862, p. 414°. . 
Hemisinus petenensis (Tristram), Brot, Matériaux fam. Mélaniens, ii. p. 51°. 
Hemisinus zoster, Brot, op. cit. i. p. 62 (1862) *°. 
Shell oblong, with 6-7 flat remaining whorls, encircled by strong spiral ridges, 4-7 on the whorl before the 
last; last whorl rather slender, its diameter not equalling the length of the aperture. Long. 23-27, 
diam. 8-103; apert. long. 84-11 millim. 
fiab. N. GuateMaLa: Lake of Peten (Salvin §). 
KE, Guatemata: Lake of Yzabal (Morelet!; Bocourt?; Stoll®; 
Sopper, teste 
v. Lhering). 
Varies considerably in the diameter of the last whorl, which is in some exam ples 
equal to the length of the aperture (petenensis, Tristr., type spec.; Reeve, fig. 130; 
Brot, figg. 2, 2a; Fischer & Crosse, figg. 11, 11 a), in others considerably less (Reeve, 
fig. 13; Brot, fig. 1; Fischer & Crosse, t. 50. fig. 11 6): Fischer suggests that this may 
be a sexual difference. The specimens sent me by Canon ‘I'ristram from the Lake of 
Peten, and those figured by Fischer and Crosse from the Lake of Yzabal, show that 
the two forms occur in both lakes. 
