172 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



whorls. But the development of these ridges is very variable : sometimes, 

 and especially in the young shell, they are very distinct, and such speci- 

 mens represent v. Ihering's G. fracta. In a few individuals the upper 

 ridge is completely absent, even in the young shell, and this part of the 

 shell appears depressed between the row of tubercles and the peripheral 

 angulation : such individuals are mentioned by v. Ihering (p. 274) as 

 variety cuevensis. With increasing age these spiral ridges disappear, and 

 on the last whorl of large shells no trace of the upper one is found, and 

 the lower one (on the periphery) becomes indistinct. At the same time, 

 the tubercles become less pronounced, and the upper half of the whorl 

 appears evenly convex, without depression, representing thus the typical 

 G. dalli. We possess all intermediate stages. 



There is also considerable variation in the development of the revolving 

 striae, both of the upper side and of the base. In older shells they become 

 more uniform and less distinct, especially the finer ones. The larger 

 striae are often beautifully waved in crossing the tubercles near the suture. 



Record of specimens : Mouth of Santa Cruz River, 1 6 sp. ; Upper Rio 

 Chalia, 5 casts; Shell Gap, Rio Chico, upper horizon, 12 sp. (mostly casts); 

 Arroyo Gio, i cast ; Lake Pueyrredon, 600' above base, 2 casts. 



Distribution: Jack Harvey and Jegua quemada, Suprapatagonian beds 

 (v. Ih.). 



Affinities: This species has a better claim than the preceding one to be 

 compared with G. magus (L.), both having radial and spiral sculpture 

 combined ; for the rest, in the details of sculpture, general form of shell, 

 and shape of umbilicus, there are numerous differences. But we may say, 

 if there is any relation of this species, it is with Pliocene and recent forms. 



104. GIBBULA DIAMETRALIS Cossmann. 



PI. XXXI, Fig. lo"- 6 . 



1899 G. d. Cossmann, in : Journ. Conchyliol., p. 5 (of sep. cop.), pi. 10, f. 1-3. 

 Shell conical, broader than high, umbilicated. Whorls convex, suture 

 distinct. Upper surface of whorls with 2 strong, rounded, revolving ribs, 

 and a similar angulation on the periphery, which is partly exposed on the 

 upper whorls. Upper rib with very elegant crenulations or plications, 

 formed by short, oblique ribs radiating from the suture. These crenula- 

 tions are restricted to the upper part of this rib, the lower being occupied 



