400 EARLIEST KNOWN FOSSIL MAMMALIA chap. xx. 



of rocks. Ill 1832 I pointed out that the occurrence of this 

 single fossil in the oolite was " fatal to the theory of success- 

 ive development," as then projjounded.* Since that period 

 great additions have been made to our knowledge of the ex. 

 istence of land quadrupeds in the olden times. We have ascer- 

 tained that, in Eocene strata older than the gypsum of Paris, 

 no less than four distinct sets of placental mammalia have 

 flourished; namely, first, those of the Headon series in the 

 Isle of Wight, from which fourteen species have been pro- 

 cured; secondly, those of the antecedent Bagshot and Brackles- 

 ham beds, which have yielded, together with the contem- 

 poraneous "calcaire grossier" of Paris, tw^enty species; 

 thirdly, the still older beds of Kyson, near Ipswich, and those 

 of Heme Bay, at the mouth of the Thames, in which seven 

 species have been found; and fourthly, the plastic clay or 

 lignite formation, which has supplied ten sj^ecies.f 



We can scarcely doubt that we should already have traced 

 back the evidence of this class of fossils much farther, had 

 not our inquiries been arrested, first, by the vast gap between 

 the tertiary and secondary formations, and then by the marine 

 nature of the cretaceous rocks. 



The mammalia next in antiquity, of which we have any 

 cognizance, are those of the upper oolite of Purbeck, dis- 

 covered between the years 1854 and 1857, and comprising no 

 less than fourteen species, referable to eight or nine genera; 

 one of them, Plagiaulax, considered by Dr. Falconer to have 

 been a herbivorous marsupial. The Avholo assemblage appear, 

 from the joint observations of Professor Owen and Dr. Fal- 

 coner, to indicate a low grade of quadruped, probably of the 

 mai'supial type. They were, for the most part, diminutive, 

 the two largest not much exceeding our common hedgehog 

 and polecat in size. 



« Principles of Geology, 2d ed., i. f Lyell's Supplement to 5th ed. of 



173. Elements. 1S57. 



