MOLLUSC A OF PORTO RICO. 
433 
Family RISSOID^E. 
Surface reticulate. 
Genus RISSOA Freminville, 1813 
Section ALVANIA Leach, 1826. 
Rissoa epima, n. sp. 
Shell small, subtranslucent, pinkish white, with a touch of purple brown on the pillar lip, 
reticulated, five-whorled; two nepionic whorls whitish, smooth, not polished; subsequent whorls with 
narrow, hardly flexuous, rounded axial ribs, with slightly wider interspaces extending from suture to 
suture but not over the base, and crossed above the suture by three subequal, nearly equally spaced, 
spiral threads, slightly tumid at the intersections; the posterior one nodulous at the intersections; on 
the base five simple threads, the posterior one of which marks the line of the suture and the anterior 
ending of the axial ribs; aperture rounded ovate, the lip continuous, simple, moderately thickened; 
throat not lirate; suture deep. 
Length of shell, 2; of last whorl, 1.25; maximum diameter, 1.12 mm. 
Mayaguez Harbor, Porto Rico, at station 6062, in about 25 fathoms, sand, one specimen. 
Although this species has no very strong characteristics, it does not seem practicable to unite 
it with any of the species of Alvunia known from the West Indies. It is more slender than any of the 
others, proportionally, and has a different number of spirals from any mentioned by Mbrch. 
Section THAPSIA Monterosato, 1884. 
Rissoa portoricana, n. sp. Plate 53, fig. 20. 
Shell small, thin, rather slender, bluish white, with five sculptured whorls; sutures rather deep, 
whorls convex, the nepionic smooth and inflated, the subsequent minutely spirally threaded with 
rather distant threads crossing feeble and rather irregular axial ridges, which become obsolete near the 
periphery; aperture with the margin slightly thickened ; a minute chink behind the callus at the pillar. 
Length, 3; maximum diameter, 1 mm. 
Mayaguez Harbor, Porto Rico, at station 6062, in 25 fathoms, one specimen. 
A small and inconspicuous species, but unlike any other yet known from the region. 
Genus RISSOINA d'Orbigny, 1840. 
Shell rissoiform, turrieulated, with numerous whorls which are somewhat convex; apex mani- 
millar; aperture oval, semilunar; lip thickened within and without, canaliculate or subcanalieulate at 
the base; operculum oval, elongate, horny, paucispiral, with an excentric nucleus; its internal face 
bearing a club-shaped appendage. 
Rissoina decussata Montagu. 
Turbo decussatus Montagu, Test. Brit., p. 399, 1803. 
Rissoiiurmecussata Tryon, Man., ix, p. 385, pi. i.vni, figs. 15-17, 1887. 
Shell with about twelve nearly flat whorls, with an elevated, sharp spire, sculptured with from 
twenty-five to twenty-eight delicate, longitudinal riblets, which are crossed by very fine revolving 
striae; outer lip very heavy; aperture almost semilunar; columella nearly straight, 'somewhat excavated 
in the middle; basal canal feeble. Color white. 
Length, 6; diameter, 2.5 mm. 
San Juan Harbor, Porto Rico; several broken specimens. 
Tryon’s figures do not accurately represent the aperture of this species. It was erroneously 
described as a British species by Montagu. 
Rissoina browniana d’Orbigny. 
Rissoina broivniana d’Orbigny Moll. Cubana, p. 28, pi. xii, figs. 33, 35, 1842. 
Rissoa lievissima C. B. Adams, Contr. to Conch., 1850, p. 115. 
Shell smooth, with eight nearly flat, whorls, highly polished, slightly curved; apex acute; aperture 
rather small; outer lip much thickened. Color white, sometimes with two or three pale hands, which 
are, however, quite evanescent. 
Length, 4; diameter, 1.5 mm. 
Mayaguez, Porto Rico; two shells. 
K. C. B. 1900— 28 
