MOLLUSCA OF PORTO RIOO. 
415 
whorl above its suture, and a second very pale one, which is almost obsolete, of about the same width 
on the middle of the base, which is best seen within the aperture by transmitted light; nuclear whorls 
two and one-half, helicoid, somewhat oblique, moderately prominent; the periphery of the last whorl 
projecting but slightly beyond the outline of the first post-nuclear whorl, not immersed but sur- 
mounting the later whorls, their axis being almost at right angles to the axis of the post-nuclear whorls. 
Post-nuclear whorls ten, flattened, slightly contracted at the sutures, traversed by strong, regular, more 
or less vertical, axial ribs, of which about fourteen appear upon the second, eighteen upon the fifth, 
twenty upon the seventh, and twenty-six upon the ninth whorls; these ribs pass almost undiminished 
in size over the periphery and base of the last whorl to the umbilical region. Intercostal spaces orna- 
mented by six equally spaced, deep, broad spiral striations, which extend partly upon the sides of the 
ribs, but do not reach or cross their summits. Base similarly ornamented with six spiral st riations which 
are placed much closer. Aperture subovate, somewhat effuse at the junction of the oblique, slightly 
revolute columella with the strongly curved outer lip, which meet almost at right angles; a well- 
marked callus covers the parietal wall within the aperture and connects the outer lip at the posterior 
angle with the columella. Columella provided with a quite prominent oblique fold near its insertion; 
outer lip pellucid, showing the external sculpture and color bands within. 
Length, 4.7; length of spire, 4.3; diameter, 1.2 mm. 
The type and three specimens are in every respect identical and are from Mayaguez, Porto Rico. 
Turbonilla insularis, n. sp. Plate 53, fig. 21. 
Shell of medium size, moderately stout, of almost rectilinear outline, milk white, with the upper 
wiiorls stained dusky. Nuclear whorls decollated; a small portion only remains, whose size would 
indicate a rather large nucleus. Post-nuclear whorls thirteen, very slightly convex, almost flattened, 
with about sixteen prominent more or less vertical, axial ribs on the fifth, eighteen on the tenth, twenty 
on the eleventh, and twenty-eight on the t welfth whorl, which pass over the well-rounded periphery of 
the last whorl, becoming feebler upon its base, finally vanishing just before reaching the umbilical 
region. Intercostal spaces broad and undulating, rather more than double the width of the ribs, 
except upon the twelfth whorl, which has a decided increase in the number of ribs and a consequent 
diminution in the breadth of the intercostal spaces; these spaces are traversed by about eleven quite 
regularly spaced, deep, spiral striations on the fifth whorl and twelve upon the succeeding volutions, 
where there is a tendency toward further division of the spaces between these striations by an occa- 
sional intercalation of a less pronounced line; these spiral lines do not pass over the axial ribs, but are 
confined to the troughs of the intercostal areas. Base traversed by about eight faint, reddish, wavy, 
spiral lines. Sutures well defined, rendered crenulate by the prominent summits of the ribs. Aperture 
irregularly rhomboidal, showing the external sculpture within. Outer lip gently curved, somewhat 
effuse at its junction with the twisted, oblique, and revolute columella. 
Length, 7.1; length of spire, 6.4; diameter, 1.8 mm. 
Locality, Mayaguez, Porto Rico. 
Superfamily NU CLEOBRAN CHIAT A. 
Family ATLANTID7E. 
Genus ATLANTA Lesueur, 1817. 
Shell fragile, transparent, discoidal, compressed, with a prominent carina; nucleus dextral; 
aperture oval, straight, deeply sulcate above at the carina; operculum subtriangular, bearing an 
apical nucleus. 
Atlanta peronii Lesueur. 
Atlanta peronii Lesueur, Journ. de Phys., lxxxv, pi. ii, fig. 1, p. 390, 1817. 
Shell discoidal, with numerous earlier whorls, which are elevated into a low spire on the upper 
side; later whorls openly coiled but connected by the carina; carina strong, obliquely striate; surface 
with delicate growth lines, glassy. 
Greatest diameter, 12; of aperture, 5; lesser diameter of aperture, 3 mm. 
Aguadilla; Boqueron Bay, Porto Rico; one shell from each locality; widely distributed. 
