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or margin of ovopositor. In ovopositor of immature females (figs 247, 252) only that smali thickness present, but narrower.

Distribution. Lower Amazon Basin.

Measurements (in mm). 50.8 by 19.0; 40.4 by 18.5; 46.3 by 18.7.

Materiał examined. Brazil; Amapś; north of Porto Grandę, Araguari River, 10 specimens, MZSP 28290 (INPA; Magalhaes, C. co).; 18/viii/l 992).

Genus Pachychilus I. & H. Lea, 1850 (Type species: Melania graphium Morelet)

Pachychilus sp.

(Figs 15, 16,61,95,96,265-281)

Description.

Shell (figs 15, 16). Ltmg, turriform, up to nine whorls. Color elear beige with broad axial bands dark brown, irregular, of imprecise borders; apex elear, other elear narrow band in suture. Each whorl little convex, almost straight outline; suture inconspicuous. Surface almost smooth, five last whorls sculptured by narrow and Iow spiral lines, discretely irregular. Aperture simple, canal weak. Protoconch present, of two convex glossy whorls.

Head-Foot (figs 265-267). Color consisting by transversal bands intercalated brown and beige, of imprecise limits; posterior regions and foot sole beige. Head protruding, re!atively large. Snout large, broad, bilobed anterior margin with several obliąue furrows. Mouth longitudinal, in anterior snout cxtremity. Tentacles stubby, long. Ommatophore sessile in head, situated in ventral-outer region of tentacles base. Eyes dark, situated in a smali depression of central-dorsal region of each ommatophore. Foot of medium size, without divisions; anterior furrow of pedał glands. Ovopositor in right-dorsal region of foot, described bclow. Columellar muscle of about one whorl.

Operculum (fig. 61). Palcispiral, circular, nucleus discretely excentric, occupies entire aperture. Scar very long, elliptical, near inner margin.

Mantle organs(figs 268,270, 271). Mantle border simple, thick, without pigment except very smali, circular, brown, sparse spots. Two flaps well differentiable in right extremity, anterior to anus and genital duets. Siphon inconspicuous. Osphradium long, ridge-likc, with about half of gili length; anterior end posterior to that of gili. Gili narrow and long, about same length than pallial cavity; filaments triangular, Iow; tip rounded, almost central. Ctenidial vein narrow. Between gili and rectum a relatively narrow area. Hypobranchial gland inconspicuous. Adrectal sinus broad, rectum running free within it. Rectum broad, with several fecal pellcts obliąuely disposed. Anus siphoned, posterior from mantle border. Genital duets in right margin described bclow.

Circulatory and excretory Systems (figs 270, 272, 273). Heart characters similar to those of preceding species, with aortas attached to anterior region of style sac. Kidney of about 1/4 whorl, almost entirely solid; about half exposed in pallial cavity and half in visceral mass between pericardium and rectum. A chamber preceding nephrostome with left surface smooth and right surface attached to rectum, with two longitudinal and broad folds, between both folds a relatively dcep furrow. Nephrostome, as in Doryssa preceding species, broad, between rectum and kidney, and protected at left by a flap inserted in outer lamina of pallial gonoduct. Some Iow and well spaced transversal folds in region of nephrostome, and anterior to it, in face with rectum.

Digestive system (figs 267-270, 272-280). Buccal mass of medium size, situated almost entire within snout. Jugal muscles mainly distributed in dorsal region. Pair of mló present but narrow. Jaws of medium size and dark brown in color. Outer surface of dorsal buccal mass wali with a pair of semi-circular, dark brown spots (fig. 274: ds). Inner surface of this wali with a pair of longitudinal, broad folds, beginning in jaws; each fold presents a central, longitudinal furrow in they broadest region; both folds continue in esophagus. Duet of salivary glands in outer and posterior margin of these folds. Odontophore distinctive features (figs 275-278): 1) m2 narrow; 2) ml5 absent or inconspicuous; 3) m7 originated from anterior-median-dorsal region of m4 and not from subradular membranę; 4) mil pair similar to that of Doryssa atra, but with only a pair of connections with outer side of radular sac near its insertion; 5) a fusion of m5 borders with subradular membranę; 6) m5 inserted in radular ribbon by large and long area; 7) m8 very long and narrow. Radula very long and intensely coiled. Subradular caitilage with a long ventral projection in each side, parallel to tissue on radula (to). Radular teeth (figs 95, 96):