5 68 



PATAGONIAN EXPEDITIONS I ZOOLOGY. 



those of P. rushii, the difference in the figures being chiefly due to the 

 side teeth being shown in their natural positions, while in P. rushii they 

 are drawn from a frayed radula, as is usually done in allied genera. 



The teeth of a form from the Santa Maria River, a tributary of the 

 Sinos, identified as P. lapidum (probably not that but an allied species) 

 have been figured by Dr. H. von Ihering, as having two basal teeth on 



FIG. 15. 



B 



Potamolithus lapidum supersulcatus, A, the teeth of a half-row in their normal positions. B, cen- 

 tral and outer marginal teeth of another individual. 



each side. The figure is somewhat diagrammatic, but shows teeth re- 

 sembling P. I. siipersulcatus. 



The eggs are deposited in plano-convex chitinous capsules about .6 

 or .7 mm. in diameter, adhering to shells and probably stones. The 

 embryonic shell is smooth, Naticoid or globular in probably all of the 

 species. So far as I can see, it is quite continuous with the neanic stage. 

 In all of the species studied, the earlier portion of the neanic stage is also 

 Naticoid. In some forms this shape persists to maturity, but in others 

 angles or carinae set in, their appearance dividing the period of youth into 

 two or three substages ; so that a highly specialized form may pass suc- 

 cessively through rounded, singly carinate, bicarinate, tricarinate, and 

 finally varicose stages. The degrees to which these sculpture-conditions 

 are accelerated and the stage finally reached, allow us to fix the relation- 

 ships and evolutionary grade of the several forms with some degree of 

 accuracy, in species where the young stages are accessible. 



These little river snails live on and under stones, at and below low- 

 water mark, often in copious numbers. Up to this time, they have been 

 collected at comparatively few places, yet the range of the genus probably 



