PILSBRY: NON-MARINE MOLLUSCA OF PATAGONIA. 567 



fied as P. lapidum} In this form "the penis is inserted nearly on the 

 median line of the back of the neck (fig. 13). It was not found for a long 

 time, because the number of females far exceeded the males among the 

 animals examined. It is very broad at the base, provided with an obtuse 

 hump and runs somewhat coiled towards the right side, where its summit 

 lies near the base of the tentacle. It is perforated throughout by the vas 

 deferens, and terminates in a slender conical point, which is encircled by 

 a sort of prepuce." 



Dentition of Potamolithus. I have examined the teeth of P. rushii 

 and P. lapidmn super sulcatits. The former (fig. 1 4) has teeth with the formula 

 6 -^. 10. 33. 40. The middle cusp of the central tooth is long but rather 



FIG. 14. 



Potamolithus rushii, A, the teeth of a half row, somewhat pressed backward, foreshortening the 

 cusps, especially of the marginal teeth. B, central and cusp of the admedian teeth, in their nor- 

 mal positions. 



narrow, and the side cusps are small. The cusps of the admedian tooth 

 are of about equal width, but are longer in the middle, as shown in fig. 

 14, B. On the inner marginal tooth the cusps are extremely small and 

 numerous, and on the outer they are still more numerous. 



In P. lapidmn supersulcatus from Fray Bentos on the Uruguay River 

 (fig. 1 5) the formula is ^ 5 . 8(4, 1,3.) 17. 1 8, or in another radula, the central 

 tooth has the cusp-formula ^ 5 . All of the teeth have much larger cusps 

 than in P. ntshit, and on the admedian tooth the median cusp is much 

 longer than its fellows.' All of the teeth have the same general shape as 



1 Malakozoologische Blatter, n. F., VII, 1885, pp. 96-99. 



