ODOXTOSTOMU!*. 93 



small folds. The parietal lamella is angular and twisted, composite, 

 being formed by the union of three lamella? ; and there is usually a 

 transverse barrier standing behind the lower palatal fold. The jaw 

 is plaited. 



Distribution, Argentine Republic and Uruguay, chiefly in the La 

 Plata drainage. 



This subgenus differs from the other groups of Odontostomus in its 

 short, compact shape, and the composite parietal lamella, which con- 

 sists of three lamella} the angular, parietal and ihfraparietal, 

 united into one angular, outwardly trih'd tooth. In pi. 15, fig. 33, a 

 young specimen of 0. itrtt/<its is figured, showing the separation of 

 the three components of the parietal tooth, at a particular stage of 

 individual development. In retaining these three lamellae, Pluyio- 

 dontes approaches Tomigerus; and as in that genus, it will be 

 noticed that the infraparietal lamella is the longest, the true parietal 

 being reduced. The liasai and lower palatal folds are comparatively 

 small, but the columellai lamella is far more developed than in 

 Tomigerus, being the large>t tooth in the aperture. No other 

 Odontostomus has the transverse harrier standing within the series 

 of lip-folds, which is possessed by all the Playiodontes except 

 patdgonicus. 



The teeth of the species are very similar throughout the subgenus, 

 except in the 0. patogonicrts, in which they are partially degen- 

 erate. 



In 0. dentatus (pi. 15, fig. 25) and 0. patagonicus the apex has 

 very fine, waved striae, faintly decussated by spiral lines. In 0. 

 dadakus (pi. 15, fig. 32), and probably all the other species, it is 

 finely costellate. 



Key to Species of Plagiodontes.. 



1. Aperture almost closed by 7-10 teeth and a transverse plate be- 

 hind them. 



a. Surface merely wrinkled with growth lines ; clear corneous. 

 Uruguay, Entre-rios. dentatus, p. 94. 



a 1 . Surface distinctly and closely stria te. 



b. Strife almost rib-like; opaque-brown, whitish behind 

 the aperture. S.-E. Argentina. rocce, p. 97. 



b l . Striae fine and close. 



c. Shell obese, whitish, the sutures moderately im- 

 pressed, dcedaleus, p. 97. 



