PREFACE. xxxi 



terminal series of vertebra?, is imsymmetrical. The process of 

 differencing which leads to the ' homocercal ' type begins in the 

 mesozoic period and prevails in the neozoic. (See Table of 

 Strata, &c, p. xxxviii.) 



A corresponding modification of the caudal vertebra? prevails 

 in neozoic birds ; but the embryos of the existing species shoAv 

 the terminal vertebra? distinct, in a tapering series, before they 

 are massed into the ' ploughshare bone ;' and such, doubtless, was 

 the law of development in all the extinct species which have left 

 tertiary ornitholites. But the earliest and as yet sole evidence 

 of the fossil skeleton of a mesozoic bird shows the retention of 

 the embryo condition, with ordinary growth of the vertebra?. 1 



Modern ruminants are hornless when born, and have the me- 

 tapodials supporting the phalanges of the cloven foot distinct ; 

 at an earlier foetal period rudiments of upper fore-teeth start in 

 the gum but do not get beyond it. The eocene mammal that first 

 indicates the ruminant type retained the transitory, and developed 

 the aborted, characters of its successors. The metacarpals and 

 metatarsals never coalesced to form a ' canon-bone ;' the upper 

 canines and incisors were functional, but small and equal-sized ; 

 and, as horns never sprouted, Cuvier called the extinct beast 

 ' weaponless' (Anoplotherium). In modern horses the digit on 

 each side the one supporting the hoof is undeveloped, and is 

 represented by a concealed rudiment of the metapodial called 

 ' splint-bone.' In the miocene horses these metapodials reached 

 their full length and supported hoofed digits, but of small size, 

 like the ' spurious hoofs ' of the ox. The eocene mammal initia- 

 ting the type had these hoofs so developed as to form a functional 

 tridactyle fool. Moreover, in the Palceotherium, certain teeth 

 (symbolised in the present Work as p 1 ) which are rudimental and 

 deciduous in the horse, were persistent and functional. The mesozoic 

 marsupials manifested a lower or less differenced state of dentition, 

 either by the degree of sameness of form (Phascolothere), or by the 

 superior number (Thylacothere) of the molar series of teeth. 

 1 Philosophical Transactions-, 1863, pp. 33, 45, pis. I. and III. 



