ORTMANN I TERTIARY INVERTEBRATES. 289 
The identification of Nos. 3 and 5 has changed, No. 4 is entirely doubt- 
ful, so there would remain only three species. 1 
Our list of living forms is the following : 
Cellaria fistulosa. 
° 0 Aspidostoma giganteum. 
Heteropora pelliculata. 
Rhynchonella squamosa. 
° Magellania lenticularis. 
0 0 Terebratella dorsata. 
Mytilus cf. chorus. 
0 0 Mytilus magellanicus. 
0 0 Infundibulum corrugatum. 
° Infundibulum clypeolum. 
° Verruca Icevigata. 
Balanus cf. psittacus. 
This is 12 species out of 15 1, or about 8 per cent., which would agree 
fairly well with v. Ihering’s percentage. But is to be remarked that I am 
positive of the identity of only 7 species (marked 00 and °), and that I 
have compared only 4 (marked 0 °) with living individuals. 
A slight change in the systematic views of the author would change 
this percentage considerably : for it is only a matter of taste whether we 
consider the following as distinct species or as forms of living ones : 
Tennysonia subcylindrica and T. stellata. 
Rhynchonella plicigera and R. nigricans. 
Crepidula gr eg aria and C. grandis. 
Siphonalia domeykoana and A. dilatata. 
Trophon patagonicus and T. laciniatus or g ever sianus. 
We even might extend this to the different species of Voluta. 
Thus, disregarding this line of evidence, the safest way to determine 
the age of any deposit is to compare its fossils directly with those of 
other localities, the age of which has been ascertained. In trying to use 
this method we meet with extraordinary difficulties in the Patagonian 
beds ; the Patagonian fauna is very peculiar, and there are hardly any 
species that have been found elsewhere in well-known deposits. At 
any rate — as we shall see below — the only clearly marked relations are 
with fossils of the southern hemisphere, and as it happens, all these 
southern localities (Chili, New Zealand, Australia) are of doubtful age: 
the same discussion as to their age exists as in the case of the Patagonian 
beds. When we turn to well-known deposits of the northern hemisphere, 
we find that no species at all (perhaps with a few insignificant exceptions, 
Cellaria fistulosa , Heteropora pelliculata , Vermetus intortus ) are identical 
1 This is a remarkable instance of how the different opinions of different writers may change 
the conclusions drawn by this method, and demonstrates clearly the chief dangers of it. 
