SUBORDER c PERISSODACTYLA 145 



tivo V-shaped or crescentic ridges opening toward the inside. Manus tetradactyl, 

 tridadyl, or monodadyl ; pes tridadijl or monodadyl. 



The Equidae constitute a family the most abundant in forms, the most 

 highly differentiated in their extremities, and at the same time a very com- 

 plete genealogy of perissodactyls. They begin in the earlier Eocene and 

 culminate in the existing genus Equus. However great the difference may 

 now be between the modern horse and the small tetradactyl ancestral form 

 from the Eocene, the individual genera of the equine series are so closely 

 related to each other that they afford one of the most instructive examples of 

 the gradual modification and specialisation of a definite type of mammalian 

 organisation. The three subfamilies, Hyracotheriinae, Anchitheriinae, and 

 Equinae, merge from one into the other through gradual variations, and 

 represent merely developmental steps in the evolutionary series. Only the 

 fourth subfamily, Palaeotheriinae, occupies a separate position. It branched 

 off early from the Hyracotheriinae, and after differentiating into a great 

 wealth of species very soon became wholly extinct. 



The skull of the Equidae is elongated and low ; the f rontals are broad, 

 and the brain is large and strongly convoluted. The nasal bones are smooth 

 above, hornless and pointed ; they project freely above the narial opening, 

 which at times is prolonged far backward, and which is bounded below 

 by the maxillaries and premaxillaries. In the older genera, the orbits are 

 large and wide open posteriorly ; in later forms they are smaller and com- 

 pletely surrounded by a ring of bone. The lachrymals are much extended 

 on the face. The postglenoid and paroccipital processes are well developed. 



All Equidae have above and below on each side three incisors, one canine, 

 and from six to seven cheek teeth. In later forms the enamel of the incisors 

 encloses a depression (mark), which gradually disappears through wear. The 

 diastema between the conical canines and the cheek teeth increases in later 

 genera as a result of the lengthening of the facial bones. The cheek teeth 

 become progressively homoeodont, and at the same time show the gradual 

 modification of the originally short, many-rooted (brachyodont) teeth into the 

 long-crowned prismatic rootless (hypsodont) teeth, which are open below or 

 not closed until late. The superior molars are composed of four main cones, 

 two external and two internal, V-shaped in form, and, as a rule, two inter- 

 mediate tubercles varying in size and shape. In the oldest genera, the internal 

 and external cusps remain distinct, and only the intermediate tubercles are 

 connected with the internal ones by a slight ridge. In the further development, 

 not only are the external cones united into a continuous W-shaped ectoloph, 

 but likewise the internal and intermediate cusps become modified into oblique 

 and crescentic ridges. The ectoloph has three prominent buttresses, a para- 

 style in front, mesostyle in the middle, and metastyle posteriorly. The most 

 anterior of the upper premolars is always small and simple ; it is never absent 

 in the Eocene genera, yet in later forms it becomes weak and disappears before 

 the last molar breaks through. In the older species, the posterior premolars 

 are tritubercular ; in the later ones, molariform. The conformity in the pre- 

 molars and molars usually takes place earlier in the lower jaw than in the 

 upper jaw. In Owen's figure of Hyracotherium the four conical tubercles are 

 either arranged in pairs or alternately opposed to each other, and either 

 remain isolated or are united only by an indistinct transverse crest. (The 



VOL. Ill L 



