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Arquivos de Zoologia

rachidian tooth with 7 cusps, being central cusp largcr (about double), basal region with a Iow projection having a broad central elevation and a narrow elevation in each side; lateral tooth with 6 not aligned, robust cusps, being third cusp larger; maiginal teeth with 3 terminal pointed cusps, being these cusps closer in outer marginal tooth. Pair of salivary glands fused with each other and anterior to nerve ring. A narrow branch of salivary gland pass through nerve ring and newly become thick, contours esophagus to left (fig. 274). Both regions of salivary glands differ in color, anterior to nerve ring white, posterior to it beige. Anterior and middle esophagus (figs 269,274) with a pair of longitudi-nal folds in a side of a single and large pouch; this pouch beige colored with several white papillae; papillae somewhat tali and well spaced with each other. Posterior esophagus with 4-5 longitudinal folds. Stornach (figs 279, 280) with little less than half whorl. Dorsal inner surface with a sorting area differentiable in anterior-right region; two arched folds in center (concavity anterior-right); two longitudinal folds in right side, a sorting area of Iow folds around arched folds; and a smali chamber, with radia] folds, of aperture of esophagus, edged at right by a narrow fold. Gastric shield somewhat large. Central pad with about a third of ventral gastric area, bears a broad furrow in its posterior region. Crescentic ridge surrounds, at some distance, posterior region of central pad, united with this in right side. Duet to digestive gland between left extremity of crescentic ridge and central pad. Anterior gastric chamber short, with a broad, Iow fold in intestine origin. Digestive gland begins in middle-ventral region of stornach and runs 3 whorls posterior to it. Style sac of about half of remainder gastric length (fig. 272). Intestine united with style sac at almost its entire left region, separated one another by a constriction. Intestine, after style sac, contours its right side and, close to stornach, has other loop turned towards anterior (fig. 272). Rectum and anus deseribed above.

Genital system (figs 268, 273, 281). Małe and female organs similar with each other. Gonad in superior region of digestive gland. Visccral gonoducts run close columella. Pallial gonoducts almost entire opened (grooves). Outer lamina simple, thick walled, with a flap protecting nephrostome inserting in its posterior limit. Inner lamina similar to outer lamina, but with its posterior region thicker, having also in this region a sac-like chamber, somewhat short (about 1 /4 of total length of pallial gonoduct), opens in a smali orifice. Pallial gonoduct finishes posterior to anus. Female with a long, somewhat rectangular ovopositor in right-dorsal region of foot (fig. 265), with imprecise anterior margin; a furrow from aperture of pallial oviduct, and runs ventrally to antero-dorsal region of ovopositor. Ovopositor with a distinctive white color.

Measurements (in mm). 23.0 by 11.7; 21.5 by 10.7.

Habitat. In rwerunder rocks, shallow water.

Materiał examined. Mexico, Chiapas; near Palenque ruins, 3 specimens, MZSP 28285; 1 shell, MZSP 28282 (R. Pinto da Rocha & S.A. Casari col; 28/ix/l 996).

Remarks on pleurocerids. The interesting, almost smooth shell lets little doubts on the generic attribution of these last specimens, if the diagnosis of Morrison (1954) and Burch & Cruz-Reyes (1987) were used. Due lack of comparative materiał and the large quantity of species in the local, the specific identification is not given, to avoid worse error.

The thiarids and pleurocerids have few anatomical characters in common in a detailed comparison. On the other hand, conchological characters arc similar and a surę separation of both groups in conchological base is very hard to be defined, at least in South American species (as also noted Morrison, 1954: 367). The strong sculpture of examined Doryssa and the poor sculpture of the examined thiarids are not constant in each taxon, and exceptions, in both groups, are common.

The broad nephrostome of pleurocerids covered by a flap of pallial gonoducts is a distinctive character. Also, other interesting data is the chamber of inner lamina of pallial gonoducts (called seminal vesicle in małe or bursa copulatrix in female), this character surely merits further investigation. This structure maybe is not a functional part of genital system, but so from excretory system, working as an ureter. In discordance with the use of this chamber as an ureter are: 1) its site (outer lamina of pallial gonoduct); 2) lack of direct connection between this chamber with kidney chamber, separated by a thin membranę (however, a similar thin membranę separates kidney chamber from ad-rectal sinus) and