596 UNGULATA. 



are convex externally, thus differing from the corresponding tubercles of 

 the upper molars. The outer wall, therefore, presents two convex surfaces 

 separated by a depression (Fig. 312, B). The inner tubercles (vertical 

 pillars) are grooved longitudinally on their inner faces, so that in the worn 

 tooth the enamel bounding them presents an hour glass -shaped form. 

 The anterior of these inner tubercles is attached to the junction of the 

 two outer crescentic tubercles, the inner and smaller being joined to the 

 posterior part of the posterior outer tubercle. The orbit is enclosed by 

 bone and the aperture of the lacrymal canal is intraorbital. The tym- 

 panic is prolonged into a tubular meatus, the post-tympanic and post- 

 glenoidal processes of the squamosal do not approach below the latter ; 

 there is a long paroccipital. Vertebrae, C 7, D 18, L 6, S 5, C 15 to 18, the 

 centra of the cervical vertebrae are opisthocoelous. The scapula is without 

 an acromion. The ulna and fibula are reduced and ankylosed with the 

 radius and ulna. The carpus has seven bones, the trapezium being absent. 

 There are two sesamoids behind the metacarpo-phalangeal articulation 

 and one at the joint between the second arid third phalanx. The femur 

 has a third trochanter. The tarsus has six bones, the ento- and meso- 

 cuneiform being generally fused. The horse stands on the termina 

 phalanx of the middle digit which is covered with a horny hoof. The 

 so-called " knee " is really the wrist, and the " hock " is the heel. The 

 fetlock is the metacarpo-phalangeal joint, the pastern is the joint between 

 the first and second phalanges, and the coffin- joint that between the 

 second and third phalanges. The first phalanx is the large pastern, the 

 second and small pastern, and the last is the coffin bone. There is a large 

 caecum, but no gall-bladder. The interventricular septum has a cartilage 

 and there is one precava. The cerebral hemispheres are well convoluted 

 and the sulci are deep. There is a scrotum, and the female has a hymen, 

 two inguinal mammae and a bicornuate uterus. The placenta is diffuse 

 and pregnancy lasts eleven months. E. caballus L., the horse, the long 

 hairs of the tail grow from the base of their organ ; with a small callosity 

 (the chestnut) on the inner side of the leg just above the hock ; in Europe 

 and Asia wild horses were formerly abundant and they have persisted in 

 the domesticated state to the present day, but it is doubtful if there are 

 any primitively wild horses now living ; in other words the horse appears 

 to be extinct as an originally wild animal all over the world. Horses have 

 escaped from domestication and become feral in many parts of the world, 

 even in America and Australia, which were without horses when they were 

 discovered. The species is found fossil in the Pleistocene of Europe, 

 N. Asia, N. Africa and America. E. przewalskii Poliakoff, C. Asia, with 

 callosities on both limbs, but the long hairs beginning halfway down the 

 tail. Sub-genus Asinus,* asses, with the species hemionus, kiang, onager, 

 hemippus, all from Asia, and asinus the domestic ass, which probably arose 

 in Africa and was derived from the species E. africanus N. Africa. Sub-genus 

 Hippotigris H. Smith, striped, Africa, with the species grevyi, quagga 

 (probably now extinct), zebra, 'burchelli. Extinct genera Protohippus f 

 Leidy (Merychippus, U. Miocene of N. Amer. , anterior inner tubercle of the 

 upper molars connected with its intermediate 'tubercle, three-toed, the 

 outer digits not reaching the ground. Pliohippus Marsh, Hippidion Ow., 



* Landkavel, Die wilden Einhufer Asiens., Zool. Jahrb, x, 1897. Prazak, 

 Wild Horses, I, 1898. 



f Cope, Prelim. Report Vert. Pal. Llano Estacodo, Ann. Eep. Geol. 

 Survey Texas, 1892, p. 20. 



