128 UNGULATA order vii 



by the premaxillaries and frequently by the maxillaries also. In some 

 forms (Rhinoceridae), the nasal bones bear horns on rough cushion-like surfaces, 

 or these elements may be furnished with bony protuberances of various sizes 

 (Titanotheriidae). As a rule, the orbits are wide open posteriorly, and only 

 in the youngest equine genera are completely surrounded by bone. The 

 zygomatic process of the squamosal bone enters considerably into the formation 

 of the zygomatic arch. 



In its typical development, the permanent dentition of the perissodactyls 

 consists of three incisors, one canine, and seven cheek teeth in each jaw above 

 and below. This dental formula holds good for all Eocene genera. In 

 younger and more progressive forms, the modernising of the dentition brings 

 about a reduction, sometimes even a complete suppression, of incisors, of 

 upper (more rarely also of lower) canines, and of the most anterior premolars. 

 In more primitive forms, the cheek teeth are brachyodont ; in some more 

 specialised genera, prismatic. The older perissodactyls generally have hetero- 

 dont, the younger, homoeodont cheek teeth. The original quadritubercular 

 (more correctly sextubercular) crown very seldom remains absolutely 

 bunodont, the cusps being usually united by ridges (lophodont). Through 

 the intercalation of smaller outgrowths on the outer side of the ectoloph 

 (parastyle, mesostyle and metastyle) and on its inner side (crista) and on the 

 inner side of the transverse crests (crochet and antecrochet) of the upper cheek 

 teeth, the crown of the tooth becomes greatly strengthened ; in more 

 specialised forms, a covering of cement also not infrequently occurs above 

 the enamel. 



The milk dentition consists of incisors, canines and cheek teeth ; of these 

 the first two correspond with those of the permanent dentition. The milk 

 cheek teeth, on the contrary, resemble their successors only in homoeodont 

 forms ; in heterodont forms, the two posterior milk teeth are like the anterior 

 molars, and as a rule only the most anterior is characterised by simpler 

 shape. 



The vertebral column is composed of seven cervical vertebrae, twenty to 

 twenty-six dorso-lumbars, from five to six sacrals, and at least thirteen or 

 more caudals. A clavicle is wanting. The humerus is short, stout, without 

 entepicondylar foramen, and the olecranon fossa is never perforated. The 

 radius and ulna are sometimes equally developed and separate, sometimes 

 anchylosed distally. 



In the carpus, the proximal series consists of four ossicles (scaphoid, lunar, 

 cuneiform and pisiform), the distal row comprising the trapezium, trapezoid, 

 magnum and unciform. The centrale is absent. Fusion of the adjacent 

 ossicles never occurs, but through a lateral shifting of the distal series and 

 an increase in the vertical diameter, an exceedingly firm interlocking of the 

 carpal elements is brought about. The scaphoid is no longer exclusively 

 supported by the trapezoid, as in Phenacodas, but by the trapezoid and 

 magnum, and frequently the facet of the magnum is considerably larger than 

 that of the trapezoid. The lunar rests on the magnum and unciform, and 

 only the cuneiform has a single distal ossicle (the unciform) underlying it. 

 In the foi-ms with three digits subequal in length, the carpus is slender and 

 propoi'tionally high ; in the later equines with unusually stout middle digits, 

 the magnum gains considerably in extent, pushes the unciform as well as the 

 trapezoid to the side, and forces the trapezium quite out of the carpus, so 



