84 



Titanotherium Leidy. 

 Perchoerus Leidy. 

 Leptochoerus Leidy. 

 Nanohyus Leidy. 



PERISSODACTYLI. 



FAMILIES. 



I. Incisors (4? in lower jaw) in part gliriform, the outer Laving persistent 



pulps, and growing continuously in a circular direction. (Anchippodonto- 



idea.) 



ANCHIPPODONTIDAE. (XLV. a.) 



II. Incisors not gliriform. 



1. Middle digit and hoof hypertrophied and alone supporting the body, 

 the lateral (second and fourth) digits being more or less atrophied and 

 functionless, or (attypically) obsolete (reduced to the condition of 

 "splint bones"). Femur with a fossa above the external condyle. 

 Skull (attypically) much prolonged forwards. Molars subequal (not 

 decreasing forwards) and cuboid ; pre-molars (PM 3-4) also enlarged 

 (not decreasing forwards) and (except second) cuboid ; tbe second 

 (PM 2) elongated ; tbe first milk molar (I) 1) more or less persistent 

 and not replaced by a pre-molar (PM 1) ; disproportionately small. 

 Incisors with a deep invaginated fold of enamel penetrating the in- 

 terior from the crown, and producing a central cavity filled with 

 cementum. (Solidungula.) 



A. Ulna with tbe sbaft atrophied and the extremities anchylosed and 

 consolidated with tbe radius. Fibula rudimentary and anchylosed 

 to the tibia. Skull with the orbit complete behind. Upper molars 

 (PM and M) — at least, of sec/md set — with a deep valley re-entering 

 from the postrorse portion of the inner side obliquely forwards, and 

 (in connection with a more or less deep valley re-entering from the 

 introrse portion of the anterior border or the angle) more or less 

 isolating an introrse enamel lobe or area, and with two (anterior and 

 posterior) crescentic enamel islands. Lower molars (PM 2, M 2) 

 with a valley re-entering inwards from the outer wall, one from the 

 introrse portion of the anterior wall, and another (terminating in 

 anterior and posterior branches) from the posterior portion of the 

 inner wall. 



EQUIDAE. (XLVI.) 



B. Ulna with the shaft complete and moderately developed, and more 

 or less differentiated from the radius. Fibula archetypically com- 

 plete but archylosed with the tibia. Skull with the orbit incomplete 

 behind. Upper molars (PM 3-4 and M) with a deep (anterior) val- 

 ley re-entering from the middle of inner side inwards and forwards 



