IJ 8 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, n 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, 1 left valve ; Shell Gap, lower horizon, 3 single shells 
(imbedded in matrix) ; Arroyo Gio, 1 right valve ; East end of Lake 
