400 EARLIEST KNOWN FOSSIL MAMMALIA CHAP. xx. 



1832 I pointed out that the occurrence of this single fossil in 

 the oolite was ( fatal to the theory of successive development,' 

 as then propounded.* Since that period great additions have 

 been made to our knowledge of the existence of land quad 

 rupeds in the olden times. We have ascertained 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 procured ; secondly, 

 those of the antecedent Bagshot and Bracklesham beds, which 

 have yielded, together with the contemporaneous ' calcaire 

 grossier ' of Paris, twenty species ; thirdly, the still older beds 

 of Kyson, near Ipswich, and those of Herne Bay, at the mouth 

 of the Thames, in which seven species have been found ; and 

 fourthly, the plastic clay or lignite formation, which has sup 

 plied ten species.| 



We can scarcely doubt that we should already have traced 

 back the evidence of this class of fossils much farther had not 

 our enquiries 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 

 cognisance, 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 whole assem 

 blage appear, from the joint observations of Professor Owen 

 and Dr. Falconer, to indicate a low grade of quadruped, pro 

 bably of the marsupial type. They were, for the most part, 

 diminutive, the two largest not much exceeding our common 

 hedgehog and polecat in size. 



* Principles of Geology, 2nd ed. f Lyell's Supplement to 5th ed. of 



i. 173. Elements. 1857. 



