104 PATAGONIAN EXPEDITIONS I PAL/EONTOLOGY. 



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. pyrofhcrioi'itni of v. Ihering (1897, P- 3 T 5> 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- : 3) 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 : ( i ) O. pyrotherionim possesses, according to 

 v. Ihering, crenulations on the margin of the upper valve, and (2) I have 

 figured on pi. XX, fig. i^, 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. pyrotherionim. Thus two of the 

 chief characters of O. pyrotherionim 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 



