PHENACODUS. 471 



by a small metapophysis. On the twelfth the diapophysis is oval in sec- 

 tion, and the metapophysis forms the external border of the prezygapophy- 

 sis. On the thirteenth the diapophysis is short and flat, and the metapophysis 

 is elevated. On the fourteenth the diapophysis is flatter and the metapophy- 

 sis is an elongate process directed upwards and forwards. The centrum 

 of the fifth dorsal is uniformly rounded below in transverse section, and a 

 little concave anteroposteriorly. The dorsals become a little concave lat- 

 erally as we follow them backwards, until the fourteenth becomes keeled 

 below by the moderate excavation of the sides. The neural spines are 

 quite elongate and slender on the anterior part of the column, and have the 

 usual posterior slope upwards. On the twelfth the spine is first vertical, 

 •wide, and truncate at the extremity. On the thirteenth and fourteenth the 

 spines have, the posterior edge a little longer and more oblique than the 

 anterior. 



The lumbar vertebrae continue the modifications of form already com- 

 menced in the dorsals. The centra become larger and longer up to the 

 seventh and last, which is a little shorter than the first lumbar. From the 

 first to the last the articular faces of the centra increase in relative depth, 

 the sides become more concave, and the hypopophysial keel more distinct, 

 excepting on the last. Here the middle line below is more rounded. The 

 posterior articular face of the last lumbar is not flattened as is frequently 

 the case, but is but little wider than deep. The diapophyses are flattened 

 and rather thin, and they originate from the anterior two thirds of the side 

 of the centrum. The metapophyses are much elevated on all of the lum- 

 bars excepting on the last, where they form only a prominent keel. The 

 neural spine is preserved on the fourth lumbar. Its base is longer than on 

 the dorsal vertebrae, is directed forwards as well as upwards, and the ex- 

 tremity is rounded, not truncate. The neural spine of the seventh lumbar 

 is more nearly vertical and its sides are parallel. 



The three sacral vertebrae are thoroughly coossified. The diapophyses 

 form a narrow uninterrupted border, which is decxirved and expanded on 

 the anterior vertebrae. The centra are rounded below. They diminish 

 much in diameter posterior to the anterior or lumbar articular face. The 

 distal articular extremity of the third and last sacral is only half the linear 



