744 



THE AMERICAN NATURALIST [Vol. XLI 



The Dorsal Vertebrae. — Although the number of the dorsal 

 vertebrae in Moropus is less (there are thirteen present and two — 

 the eighth and eleventh dorsals — lost in the specimen under con- 

 sideration) they are perhaps more suggestive of those of Aceratheria 

 or Metamynodon from the Oligocene than those of the horse. The 

 first dorsal in Moropus has a relatively longer neural spine than 

 that in the horse, and in this respect it resembles more nearly some 

 of the more primitive perissodactyls. The seventh, ninth, and 

 tenth dorsal vertebrae in No. 1604 have complete neural spines. 

 It is seen that the neural spines of the latter vertebrae are relatively 

 shorter and have a more backward slope than in the horse. The 

 thirteenth and fourteenth dorsals have wide neural spines, which 

 are constricted antero-posteriorly at their bases. In these verte- 

 brae there are strong metapophyses. The fifteenth dorsal vertebra 

 in Moropus is quite characteristic and is different from all preced- 

 ing vertebrae. The posterior zygapophyses have already become 

 convex in the same manner as in the lumbar region. 



The transverse process is pierced at the base by a large foramen 

 and is further characterized by having a su[)erior and an inferior 

 division. The superior division of the transverse process is the 

 larger of the two and is directed outward, while the smaller inferior 

 division has a downward and backward direction, between the 

 two divisions there is a dee})ly etiiargiiiated area, which is con- 

 verted into a thiti l)()iiy l)n(igo hounding the foramen referred 

 to above. On this and the siucccdinu- \(>rtebra (the first hiinbar) 

 the metapophyses are the heii\i('.sr niid they rajndly decrease in 

 size on the succeeding lumbars, wliile on the dorsals there is still 

 a very small metapoj)liysial [)r()tuberance left on the sexenth 

 vertebra. 



Only the first three lumbar ^eHebrac are present in Vo. 1()04, 

 the specimen undtM- considerjition. but from other niateriai found 



zygapophyses are simply ,-,.n\r\ an.l ((.nca\r uith no irl-liiional 

 superior articular face- The ti;ui-\. rM> i)roc('>M'- arc onl^ nio<l- 

 erately developed. 



The Sacrum.— The siu rntn i Xo. \7()i\) is composed of four 



