THE LUMBAR VERTEBRA. 85 



The Lumbar Vertebra. 

 (Plates XXV and XXVI.) 



The last four lumbar vertebra^ of the type specimen of Dinoceras 

 mirahUe (number lO.'JG) are preserved, and are figured in Plates XX^^ and 

 XXVI. The centra are ])ro|)Oi'tionally much longer than in the mastodon, 

 and the most anterior of tlie four is excavated at the sides, and keeled 

 below, so as to be nearly triangular in a vertical section near the middle. 

 The excavation becomes less decided in the succeeding vertebra^ whicli 

 are more nearly circular in section. The keel disappears upon the last 

 lumbar, which has its under surface strongly roughened, especially near 

 the posterior margin. 



The epiphyses of all these vertebrfe. are imperfectly united with the 

 centra, and are deficient in ossification near the center, forming only thin, 

 and, in some cases, narrow rings of bone. The deficiency of ossification 

 increases the apparent concavity of the artieidar faces of the centnmi. 

 These faces akso become more oval transversely on the posterior vertebra^, 

 and especially upon the last lumbar vertebra. 



The neural canal in all the lumbar vertebrae is large, but has been 

 considerably reduced, in the vertebra of this specimen, by crushing-. 



The zygapophyses in the lumbar region are strongly articulated 

 together. The pre-zvgapophyses present a curved surface, bending 

 around, and firmly grasping, the semi-cylindrical post-zygapophyses, 

 which are inserted into them. This form of articulation is found among* 

 living Artiodactyls, and existed also in the Oreodons of the Miocene ; 

 while in the Perissodactyl group, and in Proboscidians, the zygapophyses 

 of this region are loosely applied to each other by nearly flat surfaces. 



The keel on the under surface of the centrum is stout and short on the 

 second vertebra from the sacrum, where it first appears in this specimen, 

 and is strongest on the anterior part of the centrum. On the next 

 preceding vertebra, it is somewhat thinner, Inxt more elongated, extending 

 along the who^le under surface of the centrum. On the fourth vertebra 

 from the sacrum, it becomes thin, and even sharp in front. 



