28 PEOFESSOE OWEN ON THE CHAEACTEES, ETC. 



caecum, which equally evince the mutual affinities of the odd- 

 toed or perissodactyle hoofed quadrupeds, and their claims to be 

 regarded as a natural group of the TJngulata. The placenta is 

 replaced by a diffused vascular villosity of the chorion in all the 

 recent genera of this order, excepting the little Hyrax, in which 

 there is a localised annular placenta, as in the Elephant. But 

 the diffused placenta occurs in some genera of the next group, 

 showing the inapplicability of that character to exact classification. 

 Many extinct genera, e. g. Coryphodon, Pliolophtis, Lojphiodon, Ta- 

 pirotheriti/m, FalceotJierkmt, Ancitheriimi, Hipparion, AcerotJierinm, 

 ElasmotJierivm, &c., have been discovered, which once linked to- 

 gether the now broken series of Perissodactyles, represented by 

 the existing genera Hhinoceros, Hyrax, Tapirus, and Equus. 



In the even-toed or 'artiodactyle' Ungulates, the dorso-lumbar 

 vertebrae are the same in number, as a general rule, in all the 

 species, being nineteen. The recognition of this important cha- 

 racter appears to have been impeded by the variable number 

 of moveable ribs in different species of the Artiodactyles, the 

 dorsal vertebrae, which those ribs characterize, being fifteen in 

 the Hippopotamus and twelve in the Camel. And the value 

 of this distinction has been exaggerated owing to the common 

 conception of the ribs as special bones distinct from the ver- 

 tebrae, and their non-recognition as parts of a vertebra equiva- 

 lent to the neurapophyses and other autogenous elements. The 

 vertebral formulae of the Artiodactyle skeletons show that the 

 difference in the number of the so-called dorsal and lumbar ver- 

 tebrae does not affect the number of the entire dorso-lumbar 

 series : thus, the Indian "Wild Boar has d. 13, I. 6=19 ; the 

 Domestic Hog and the Peccari have (?. 14, Z. 5=19; the Hip- 

 popotamus has d. 15, I. 4=19 ; the Gnu and Aurochs have d. 14, 

 Z. 5=19 ; the Ox and most of the true Euminants have d. 13, 1. 6= 

 19; the aberrant Euminants have d. 12, I. 7=19. The natural 

 character and true affinities of the Artiodactyle group are further 

 illustrated by the absence of the third trochanter in the femur, and 

 by the place of perforation of the medullary artery at the fore and 

 upper part of the shaft, as in the Hippopotamus, the Hog, and most 

 of the Euminants. The fore part of the astragalus is divided into 

 two equal or sub-equal facets : the os magnum does not exceed, or 

 is less than, the unciforme in size, in the carpus ; and the ectocunei- 

 form is less, or not larger, than the cuboid, in the tarsus. The 

 digit answering to the third in the pentadactyle foot is unsymme- 

 trical, and forms, with that answering to the fourth, a symmetrical 



