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. Mytilus cf. chorus. 



Aspidostoma giganteum. Mytilus magellanicus. 



Heteropora pelliculata. Infundibulum corrugatum. 



Rhynchonella squamosa. Infundibulum clypeolum. 



Magellania lenticularis. Verruca Icevigata. 



Terebratella dorsata. Balanus cf. psittacus. 



This is 12 species out of 151, 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 ' and ), and that I 

 have compared only 4 (marked c ) 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. stcllata. 



Rhynchonella plicigera and R. nigricans. 



Crepidula gregaria and C. grandis. 



Siphonalia domeykoana and .S. dilatata. 



Trophon patagonicus and T. laciniatus or geversianus. 



We even might extend this to the different species of Valuta. 



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. 



