ODONTOSTOMUS. 93 



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

 being formed by the union of three lamellae ; 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 Odontostonms in its 

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

 sists of three lamellae the angular, parietal and infraparietal, 

 united into one angular, outwardly trifid tooth. In pi. 15, fig. 33, a 

 young specimen of 0. denfatus 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, Plagio- 

 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 basal and lower palatal folds are comparatively 

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

 Tomigerus, being the largest tooth in the aperture. No other 

 Odontostomus lias the transverse barrier standing within the series 

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

 patagonicus. 



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

 except in the 0. patagonicus, 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. 

 dadaleus (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 striate. 



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

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



b l . Striae fine and close. 



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

 pressed, dcedaleus, p. 97. 



