102 
PATAGONIAN EXPEDITIONS ! PALAEONTOLOGY. 
The present writer, in 1897, recognized first the difference of this oyster 
of the Patagonian formation from the true O. patagonica of d’Orbigny, but 
he made the mistake of regarding the more elongated form from the 
upper Rio Chalia and the more circular form of Santa Cruz as different 
species, and described them under the names of O. philippi (substituting 
this name for the preoccupied bourgeoisi of Philippi) and O. hate fieri re- 
spectively. In the same year, a few weeks later, v. Ihering called the 
more rounded form of Santa Cruz by the name of O. percrassa (preoccu- 
pied by Conrad for a Miocene species of N. America, see : Whitfield, 
^94, p. 29, pi. 3, f. 1-4), giving as its chief characters the widely remote 
lamellae formed by the lines of growth, which gives a terraced or gradu- 
ated appearance to the external surface. This same character is found in 
the type-specimen of my O. hate fieri ; but I do not regard it as of specific 
value. Some individuals show it very distinctly developed, but in others 
we have all transitional conditions to the crowded lines of growth of what 
I have called O. philippic and, indeed, both characters — the distant and 
the crowded lamellae — may be present in one and the same individual at 
different stages of age (see pi. XV). Moreover, this percrassa-stage — 
which is apparently due to very vigorous growth of the shell — is not re- 
stricted to the locality of Santa Cruz or to the particular horizon of the 
Patagonian beds represented there. 
Very recently (1899, p. 8), v. Ihering has again discussed the oyster of 
Santa Cruz, adopting the specific name of hate fieri. Here he does not 
consider the graduated appearance of the shell as of specific value, but 
still he maintains its specific difference from “ patagonica ” = phiiippii Ortm. 
The chief characters given are : 
1. Very thick shell and rounded outline. 
2. Short and broad area. 
3. Ligamental fossa of upper valve concave or flat, not prominent on 
the lower margin of the area, or only slightly so. 
4. Muscular impression far distant from the margin. 
Characters (1) and (2) have been discussed above, and are of no specific 
value : we have at the type-locality elongate shells as well as rounded 
ones, and individuals with an elongated area. These characters change 
sometimes in the same individual with age (see pi. XV). In fact, the 
typical O. hatcheri (or percrassa) is not the only form found at Santa 
Cruz, and it is not even the prevailing form (see : No. 1 under record of 
