Peterson: The Osteology of Promi<:rycochcerus. 



177 



neural spines arc lower, but with a greater antero-posterior diameter, 

 and greater forward inclination than in Merycoidodon. 



The Sacrum {Cf. Figs. 28, 29, 30). — In the type-specimen. No. 1080, 

 and also in its companion skeleton No. 1079 the sacra have seven 

 coossified vertebrae; specimen No. 1081, also one of the three originals, 

 has eight (see Fig. 28) ; on the other hand in Merycoidodon culbertsoni 

 there are usually but four. The sacrum of the species under descrip- 

 tion is very robust and its antero-posterior diameter is nearly twice 

 that of its transverse. It extends back of the ischial tuberosity and 

 the lateral mass, or united transverse processes, approach close to the 

 upper borders of the ilium, thus forming a nearly complete arch 

 over the long and comparatively narrow pelvic cavity. The ventral 

 face of the centrum of the first sacral is slightly keeled and a rugose 

 band across the body indicates the line of coalescence with the suc- 

 ceeding vertebrae. The sacral foramen is large, helping to form the 

 arch lateral to the postzygapophysis. The latter coalesces with the 

 prezygapophysis of the succeeding vertebra, the union being marked 

 by a rough suture, while the united articular processes further back 

 show little or no separation from one another in old individuals. The 

 neural spine of the first sacral is separated from the succeeding spines 

 by a considerable space. The pleurapophysis is very heavy and forms 

 the principal support for the ilium, while that process of the second 

 sacral is much smaller, though its entire lateral mass is also in contact 

 with the ilium. The neural spines of 

 the second, third, and fourth sacrals 

 coalesce, are moderately high, arid taper 

 gradually backward. The united trans- 

 verse processes form a broad plate of 

 bone, curving upward at the lateral 

 borders and perforated by the dorsal 

 foramina. The transverse processes of 

 the last three sacrals are more or less 

 separated while the articular processes 

 and the centra are firmly united. The 

 centrum of the last sacral vertebra is 

 broad transversely and much flattened 



vertically; the neural spine is of moderate size and has an enlarged 

 rugosity at the summit, while the transverse process is broad antero- 

 posteriorly and is sometimes united with that of the preceding sacral 

 vertebra forming a large sacral foramen. 



Fig. 25. Dorsal and lateral 

 views of caudal vertebra* of Pro- 

 merycochcerus carrikcri. No. 1081; 

 \ nat. size, i, first caudal; 2, 

 second caudal; 3, third caudal; 

 4, fourth caudal. 



