102 PATAGONIAN EXPEDITIONS : PAL/EONTOLOGY. 



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. pJiilippi (substituting 

 this name for the preoccupied bourge&isi of Philippi) and O. hatcheri 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, 

 1894, 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. hatcheri; 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. philippi, 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 hen. Here he does not 

 consider the graduated appearance of the shell as of specific value, but 

 still he maintains its specific difference from "patagonica" = philippii 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 (i) 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. i under record of 



