io 4 PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
* 
lack of crenulations in the true O. patagonica is always due to an incom- 
plete state of preservation, the crenulations being worn off or having dis- 
appeared by the breaking off of the margin (see below under O. patagonica) . 
0. pyrothemorum of v. Ihering (1897, P- 315, textfig. 21, numbered 20 
by mistake) seems to be a different species. It has a short, triangular 
outline, with crenulations of the upper valve. The figure is very poor, 
but since v. Ihering has kindly sent a specimen of this species to Prince- 
ton, I have verified his statement that the triangular form is very striking, 
and furthermore — as v. Ihering has already pointed out — the muscular 
impression is very peculiar, being very narrow and transversely elongated. 
The stratigraphical position of this form is said to be in the Pyrotherium 
beds, but since at present nobody knows what these beds really are, and 
since — as Ameghino himself admits (1899, p. 13) — different horizons have 
been mixed up under this designation, I cannot venture to express an 
opinion on this subject. But at any rate, a Cretaceous age of these beds 
is out of question (see Hatcher, 1900, p. 96). I may, however, call atten- 
tion to the following facts : ( 1 ) O. pyrotheriorum possesses, according to 
v. Ihering, crenulations on the margin of the upper valve, and (2) I have 
figured on pi. XX, fig. i d , an upper valve of O. patagonica , from San 
Julian, belonging to the lot described below, which has a muscular scar 
that corresponds exactly to that of O. pyrotheriorum . Thus two of the 
chief characters of O. pyrotheriorum are also found in O. patagonica , and 
it may be, that O. pyrotheriorum is only a form of O. patagonica , and that 
the two individuals, upon which this species is based, have been picked 
up just for this peculiarity in external form. If this should prove to be 
correct, its stratigraphical position would be in much younger beds (Plio- 
cene), and this would support Mr. Hatcher’s opinion, that a part at least 
of the Pyrotherium-beds of Ameghino belongs in the Pliocene. 
To sum up, we have the following results: 
1. The large oyster of the Patagonian formation is different from the 
true Ostrea patagonica of d’Orbigny, and the only, but constant difference, 
is the presence of crenulations all around the margin of the upper valve 
in the latter. Such crenulations are sometimes present near the hinge in 
the Patagonian oyster, but they are very much smaller, and found only 
' for a short distance, never all around the margin. 
2. There is only one species of large oyster in the Patagonian forma- 
tion. From our material, I may pick out individuals corresponding to 
