HATCHER : DIPLODOCUS (mAESH) l3 



paired spines commence with the sixth dorsal and are continued anteriorly in dor- 

 sals 5, 4, 3, 2 and 1, and in most of the cervicals. 



General Description and Nomenclature of the Different Vertebral 



Elements. 



A careful examination of any vertebra of Diplodocus, except it be a posterior 

 caudal or anterior cervical, will show it to consist of a centrum, neural arch, neural 

 spine, transverse processes, anterior and posterior zygapophyses, etc., or those ele- 

 ments usually met with in the vertebrae of the reptilia and higher vertebrata. In ad- 

 dition to these elements, there will be seen in the vertebrae of Diplodocus a number 

 of prominent laminae and buttresses which support the different processes and give 

 origin to certain rather deep cavities that appear as conspicuous characters on the ex- 

 ternal surfaces of the vertebrae. Owing to the extreme variation in the size, shape 

 and position, assumed by these laminae and cavities in the different vertebrae, no 

 little difficulty is encountered when a detailed description of the individual ver- 

 tebrae is undertaken. This will be facilitated by first giving a careful description of 

 those elements as they are exhibited in that part of the vertebral column where any 

 particular lamina or cavity is best represented, and by the employment of a precise 

 nomenclature for each. For those elements usually found in the vertebrae of all 

 vertebrated animals, the usual and well-established nomenclature will of course be 

 employed, while in referring to the different laminse the excellent descriptive no- 

 menclature proposed by Osborn will be used, expanding it in one or two instances 

 to meet the further requirements made necessary by our present more perfect ma- 

 terial. For the different cavities a nomenclature has been devised and will be em- 

 ployed which it is believed is both explanatory and precise and will prove to be a 

 useful descriptive adjunct. 



The Centra. — The centra throughout the entire extent of the vertebral column 

 have expanded extremities separated by very pronounced median constrictions, so 

 that each centrum is in form similar to that of an hour-glass. The centra are not 

 solid, but are composed internally of intersecting laminae arranged irregularly and 

 abutting against and supporting the thin external walls. Externally and laterally 

 each centrum is invaded by a pair of cavities, which may be called the pleuro-central 

 cavities, while inferiorly there is, except in the dorsals and sacrals, a single median 

 infracentral cavity. The centra of all the presacral vertebrae are opisthocoelous, while 

 those of the postsacrals are procoelous. They increase in length from the axis to the 

 fourteenth cervical, which is the longest in the vertebral column, and then grad- 

 ually decrease in length to the third dorsal. Throughout the succeeding dorsals. 



