nr 
the Patagonian and 7º in the Santa Cruz formation (Tro- 
chita corrugata Rve. and magellanica Gray; Trophon la- 
ciniatus Mart. and varians Orb.; Magellanica globosa Lam.) 
In general the character of the molluscan fauna of 
both formations offers with difficulty means of determin- 
ing their relative age. In this connection the Arcidae 
are always interesting which are represented in the 
older Patagonian formation by Cucullaea and Cucullaria 
and in the Santa Cruz formation by Arca and Pectun- 
culus. A single extinct genus occurs, Amathusia Phil., 
which has a striking form and has been placed by Phi- 
lippt with the Veneridae and by me, from the character 
of the hinge which has only cardinal teeth, with the 
Glycimeris having poorly developed siphons and, there- 
fore, no gaping shell. The variety Amathusia angulata 
Phil. from Santa Cruz is a very large shell, 25 em. long. 
Neither the conclusion of d'Orbigny and Darwin 
that living forms are wholly wanting, nor that of Weu- 
mayer that there exists no relation to the European Ter- 
tiary are substantiated. The large Cucullaea Dalii Th. 
corresponds very much to the C. crassatina Lam. of the 
Paris Basin and only in the latter do species of Arca 
and Cucullaria occur. The gigantic Pectunculus of the 
Santa Cruz formation is probably to be considered with 
P. pulvinatus Lam., and a further decided connection 
with the European Tertiary is to be found in the presence 
of Scutella. 
We may expect a further tracing out of this con- 
‘nection with the European Tertiary to throw considerable 
light upon the question of the age of the different Pata- 
gonian formations. Until the present this question has 
been discussed exclusively from the standpoint of fossil : 
mammais and these seem poorly suited for this purpose. 
Not only were the Ungulates and Rodents of the Pa- 
tagonian formation, from their character, placed by Ame- 
ghino in the Eocene, by Zittel in the Miocene and by 
