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rounded and tali, situated between it and style sac aperture, lies within intestine as a typhlosole (fig. 177: it). Style sac free from intestine, with length shorter than half of remainder gastric length; style present.

Genital system (figs 171, 173, 174, 182). Only females found. Pallial oviductdistinct in being entirely closed, a narrow tubę with thick walls. Other accessory tubę origins in outer-right-poste-rior region of rectum, just in a chamber replete of ova, covering extemally rectum (figs 174, 182); inserts in pallial oviduct anterior half (exact insertion variable in each specimen). Aperture of brood pouch a smali porę, sometimes papilla-like, in right region of head-foot, close columellar muscle. Brood pouch an enormous sac, dorsal to esophagus, with thin walls, free in haemocoel except for its aperture. Brood pouch posteriorly bilobed, left lobe larger and longer; esophagus runs between both lobes (fig. 173), inner surface smooth except by some sparse smali transversal septa. Brood pouch bearing morę than hundred young specimens from one to six whorls, about 80% with one whorl. Other details of genital system and number of young specimens in brood pouch see Starmuhlner (1969), Berry & Kadri (1974).

Measurements (in mm). 21.4 by 6.6; 22.2 by 7.0; 21.8 by 6.2 (figured speeimen).

Distribution. Eurasia. In Brazil apparently introduced in all main macrobasins, particularly confirmed in Parana and Amazon macrobasins, and in the Pantanal.

Habitat. Rivers or lakes in regions with some current.

Materiał Examined. Brazil. Tocantins; Araguaęu, Agua Fria river, 27 km north of Araguaęu, MZSP 28725, 11 specimens (Lima, Rocha & Furlin col., 6-12/vii/1997). Mato Grosso do Sul; Salobrinho river, MZSP 28348, 31 specimens (T. Lipparelli col. 5/95).

Remarks on thiarids. The scope of the present study is not to propose the philosophical bases for establish species in parthenogenetic animals. A pragmatic approach is taken here accepting as representative of species each lot with specimens bearing a own group of morphological characters. Howevcr, it is recognized that the aim is matter for long debates.

Aylacostoma exoplicata differs from A. ci

mainly in having a laiger osphradium, an osphradium satellite fold present, broader and shorter gili filaments, morę developed folds in rectum, simpler pallial oviduct, a large protractible fold in the brood pouch aperture and the odontophore muscle mil connected to the peri-buccal region. Despite these anatomical differences and the large geographical distance between both species, they have great conchological similarity; the shell of A. ci is has about 10 spiral cords on the penultimate whorl, in contrast to about 8 in A. exoplicata.

Both species differ from A. jaryensis (Pilsbry in Baker, 1913) in having a broader shell and the sculpture not so developed. They differ from A. sulcata (Baker, 1913) in having a narrower shell and deeper suture. They differ from A. elongata (Baker, 1913) in having a broader shell and morę sculptured surface. Therc is no other Aylacostoma with which both species can be confused.

A. exoplicata and A. ci differ anatomically from A. tenuilabris in having fewer young specimens in the brood pouch, the pallial oviduct simpler, the stornach lacking a deep furrow in the esophagus and in the duet to digestive gland apertures, posterior lobe of kidney with a pinnate glandular organization, anterior lobe of kidney not so glandular and odontophore muscle mil connected to cartilages.

The three Aylacostoma species differ from A. guaratinica (Scott, 1953) in lacking so outstanding spiral sculpture and suture, and in having morę young specimens in brood pouch.

Melanoides tuberculatus differs from the three Aylacostoma species studied here mainly in having axial shell sculpture developed, much morę young specimens in brood pouch, brood pouch larger and bilobed posteriorly, dark spots in head organized in mosaic (not in transversal bands), single fold in proximal extremity of intestinal typhlosole (in stornach), anterior lobe of kidney in mantle roof (not attached to rectum), gastric crescentic ridge attached to central pad and pallial oviduct altogether closed bearing two tubes. On the other hand, the four studied thiarids have simi-lar organization to that given by Abbott (1952) for Thiara granifera (Lamarck) from Guam.

The significanee of the above eited differences, if specific, generie or familiai levels, only will be morę secure with further morphological analysis of morę species of each group.