492 J. Starkie Gardner — Relative Ages of 



Although many of the older forms among the Siwalik mammals 

 are confined to the lower Siwaliks, yet in the iipper Siwaliks there 

 is a remarkable mixture of old and modern genera, which is else- 

 where unknown. In explanation of the circumstance that in many 

 parts of the Oriental region the existing species of mammals are 

 older than many important geographical changes, Mr. Wallace ^ 

 observes that, " It seems highly probable that in the equatorial 

 regions species have changed less rapidly than in the north 

 temperate zone, on account of the equality and stability of the 

 equatorial climate." As what is true of a species is probably also 

 true of a genus, we may assume, if Mr. Wallace's hypothesis be 

 correct, that we have here a vera causa of this peculiarity of the 

 Siwalik fauna. 



It may be added that the existing Siwalik mammalian genera are 

 remarkable for the number of species and types by which they are 

 represented. Thus Elephas contains representatives both of the 

 African and Indian sections, as well as a totally extinct section, con- 

 necting it with the Mastodons, and apparently peculiar to the 

 Oriental region. Bhinoceros likewise contains representatives both 

 of the Ethiopian and Oriental types ; and one form is doubtless the 

 direct ancestor of the existing B. sondaicus. The Siwalik Hygenas 

 contain representatives of all the known types of the genus ; and the 

 genus Sus is represented by a large number of forms, one of which 

 shows indications in its cranial characters of affinity with S. harhatvs 

 of Borneo, while in its molar teeth it presents a remarkable approxi- 

 mation to the existing Ethiopian genus, Phacochcerus. We are, 

 however, not at present in a position to say whether most of these 

 genera originated in the Oriental or the Paleearctic region. 



IV. — On the Eelative Ages of the American and the English 



Cketaceous and Eocene Series. 



By J. Stakkie Gardner, F.L.S., F.G.S., etc. 



I HAVE been desired by the Council of the British Association 

 to contribute some account of the later Fossil Floras of 

 Europe and America, with a view to determining, more precisely if 

 possible, their relative ages. 



It is within the knowledge of every one interested in geology 

 that some fourteen years ago, during the progress of the United 

 States Survey of the Territories, a number of fossil dicotyledonous 

 plants were brought by Dr. Hayden from the neighbourhood, I 

 believe, of Dakota. The exact locality is of no great consequence, 

 for anyhow they belong to the age of the Dakota series, and were 

 intercalated with beds containing marine and brackish water mol- 

 lusca of a character which cannot reasonably be assigned, according 

 to our present lights, to any later age than the Cretaceous. It was 

 startling to find Floras of such antiquity composed of dicotyledonous 

 plants at all, but especially so, to find them not betraying any marked 

 transition towards gymnospermsor even monocotyledons; but on the 

 1 " Island Life," p. 358. 



