jS southern CALIFORNIA ACADEMY OF SCIENCES. 



These earlier mammals combined the characters of many 

 of the more recent forms, and the assumption of the specialized 

 forms of today was gradual. 



The Cetaceans or Whales first appeared in the Eocene, the 

 oldest form being the Zeuglodon found in great abundance in 

 the Eocene. 



For some cause, for which no satisfactory rea,sons have 

 been assigned, California had few, if any, of these Eocene Mam- 

 mals ; nor has the question been decided as to whether we have 

 the Eocene deposits represented, or, whether there is a gap be- 

 tween the later Cretaceous and the Miocene or Middle Tertiary. 

 Eminent geologists have different views upon the subject. At 

 any rate the fact is apparent that, California has few, if any, 

 fossil remains of the Eocene IMammals sa abundant in other 

 localities. 



Possibly the Sierra Range formed an insurmountable barrier 

 to the animals of the tropical interior, or, more probably it was 

 because the Pacific shore-line skirted the base of the Sierras, 

 and consequently there were no low-lying plains nor tropical 

 marshes where such a fauna could thrive, nor interior sheltered 

 valleys to accommodate their needs. 



The extensive series of strata which form the mass of some 

 of our mountams, by many scientists referred to early Eocene, 

 are comparatively barren of fossil remains, possibly from their 

 having been deposited at great deptb, below the ocean level. 

 (See Plates i and 2, Prehistoric Fauna, which illustrate some 

 of the recent fossil Mollusca upon which the theory of the 

 Eocene age of the deposits in which they were found is based). 

 This period of rapid changes of form in animals seems to 

 have developed greater intensity as it progressed, while geo- 

 graphical divisions in the fauna and flora became more distinct, 

 and the advancement and retardation of characteristic generic 

 forms in parallel lines more stronglv marked. 



Remarkable illustrations of the progression and retrogres- 

 sion, or retardment in the evolution of contemporaneous genera, 

 their geological range and geographical distribution may be 

 taken from discoveries in Oregon and California relative to the 

 history of some of our living Land Snails. In 1865 the writer 

 discovered ? pulmonate gasteropod (Land Snail), which Dr. J. 

 G. Cooper (the able zoologist of the California State Geological 

 Survey, under Professor J. D. Whitney), to whom the shell was 

 referred, found that it belonged to an entirely different group 

 of shells from those known to occur on the Pacific Coast ; he 

 described a new sub-genus to which he assigned the snail, 

 naming it Ammonitella Yatesi, and for many years its known 

 habitat was restricted to the locality in Calaveras county, Cali- 

 fornia, where it was first discovered. In or about the year 

 1883 some fossil shells collected by the late Professor E. D. 



