Il8 PATAGONIAN EXPEDITIONS: PALAEONTOLOGY. 



individuals the number of the principal ribs does not increase materially 

 with age, but that of the secondary ribs does considerably, so that we may 

 count, in very large ones, 90-100 ribs of different size. 



Furthermore, this species varies in the development of the larger ribs. 

 In some cases (so chiefly in large individuals from Santa Cruz) the prin- 

 cipal ribs are very distinct and much larger than the secondary. A few 

 individuals from Oven Point and Darwin Station show the same char- 

 acter, but in the larger number from Oven Point this difference is not so 

 strongly pronounced, although the principal ribs are still well marked. 

 A third variety is found, to which belongs the larger number from Darwin 

 Station ; here the fasciculate appearance of the ribs, shown in old indi- 

 viduals of the typical form, begins at an earlier stage: the intermediate 

 ribs closest to the principal ones become stronger, while the principal rib 

 itself is not so strongly contrasted to them in size, and we have in shells 

 of medium size, already an appearance of the principal ribs being com- 

 posed of from two to four smaller ones. In these specimens the ribs 

 appear to be more numerous, a little finer on the average, and more 

 crowded, and they represent completely the form described by v. Ihering 

 as P. quemadensis. 



There is no sharp line to be drawn between these different forms, and 

 the form quemadensis, although not found among the larger specimens 

 from Santa Cruz, is exhibited in a few younger individuals from this 

 locality, and in a few larger ones from Oven Point. As has been stated, 

 it is the prevailing form at Darwin Station. There are all possible tran- 

 sitions between the different forms in the development of ribs. 



The outline of the shell changes with age. Young shells appear more 

 elongate, subtriangular, while larger shells are broader and more rounded 

 (see measurements). 



V. Ihering's P. fissicostalis is nothing but the cast of a larger individual 

 of this species. We have received from v. Ihering one cast under this 

 name, from Santa Cruz : it agrees completely with casts of P. geminatus 

 represented in our collection, and still connected with the shell. 



Record of specimens: Mouth of Santa Cruz River, 7 right, 11 left 

 valves; San Julian, Oven Point, 3 double, 17 right, 17 left valves ; San 

 Julian, Darwin Station, 6 double, 12 right, 7 left valves; Canon near 

 Sierra Oveja, i left valve ; Shell Gap, lower horizon, 3 single shells 

 (imbedded in matrix) ; Arroyo Gio, i right valve ; East end of Lake 



