564 PATAGONIAN EXPEDITIONS! ZOOLOGY. 



(fig. 12). It measures 5.4 mm. long, 2.4 wide, the aperture 1.8 mm. long, 

 and is composed of 6^ whorls. The apex is very obtuse. Another lot, 

 received from G. von Frauenfeld, contains shells slightly more slender 

 than that figured. In one of them I found two minute embryonic shells. 



IDIOPYRGUS gen. nov. 



The shell is perforate, solid, turrite, with long spire of very convex 

 whorls ; aperture diagonal, oval, its plane sloping forward below, posterior 

 end rounded, sinused ; a small sinus at junction of the outer lip with the 

 basal margin. Lip slightly expanded, thickened within. Operculum 

 paucispiral, with the nucleus at the lower fourth, near the columellar 

 margin. Radula having the formula ^. 7. 9. 16. 



Type, I. souleyetianus. 



The snail for which this genus is proposed differs from all known species 

 of Littoridina by its internally thickened, somewhat expanded and bisinu- 

 ate peristome, the diagonal aperture, and by having fewer cusps on the 

 upper reflection of the central teeth, as well as on the marginal teeth. The 

 scoop-like shape of the outer marginal tooth is also rather peculiar. 



FIG. 12. 



Idiopyrgus souleyetianus, half row of teeth and an isolated marginal tooth. 



Idiopyrgus has some resemblance to the Dalmatian genus Lanzaia 

 Brusina, and to the Mexican Pterides Pilsbry. In all of them the long 

 axis of the aperture stands strongly diagonal to that of the shell, the 

 posterior end of the aperture is rounded, effuse or sinused, the lip ex- 

 pands more or less, and the whorls of the tapering spire are strongly 

 convex. These apertural characters are so unusual in Amnicolidce, that I 

 am disposed to view them as indications of real relationship between the 

 three genera, rather than convergent structures in snails otherwise diverse. 



