18 MEMOIRS OF THE CARNEGIE MUSEUM 



2, The Zygapophysial Cavities. — These are four in number and in regard to the 

 position occupied by each with reference to the zygapophyses, they may be called 

 the supra- and infra-, post- or prezygapophysial cavities. They appear as conspic- 

 uous features on all the presacrals save the anterior cervicals, where they are less 

 pronounced. 



3. The Sinned Cavities. — These are quite numerous and occur with more or less reg- 

 ularity as pockets in the surface of the single or paired spines. They are not unus- 

 ually of considerable extent, as in the posterior dorsals, where they are formed and 

 partially enclosed by tlie expansion of the free edges of the various laminse that 

 spring from the sides of such spines. Again they may be small and formed by the 

 irregular development of secondary laminse, or by inflections in the external walls 

 of the spines. 



4. ' The Pleurocentral Cavities. — These are constant throughout the entire vertebral 

 column of Diplodocus, though varying greatly in the different regions in size, form 

 and structure. As the name implies, they are located one on either side of each 

 centrum, though, as frequently happens in the posterior cervicals and anterior dor- 

 sals, they may each be divided by pleuro-central laminae into two or more partially 

 distinct cavities. The pleuro-central cavities of opposite sides of the same vertebra 

 are usually separated by a thin median partition, but occasionally this partition fails 

 and they become confluent. This condition is met with more especially in the pos- 

 terior cervicals. 



5. The Infracentral Cavities. — Present on the inferior surface of cervicals and cau- 

 dals. Absent in dorsals and sacrals. 



6. The Intramural Cavities. — Present not only within the external walls of the 

 centra, but within those of the neural spines, transverse processes, zygapophyses, 

 neural arches, the different laminse, etc. ; thus these elements are reduced to a com- 

 plicated system of delicate intersecting laminse which enclose the intramural cavities 

 and abut against and give support to the external walls. They thus form a second 

 and exceedingly efficient method of combining strength and increased surface for 

 muscular attachment with lightness. 



With this general description of the different vertebral elements we may proceed 

 with a description of the individual vertebrse. 



The Cervicals. — There are fourteen cervicals represented in our skeleton No. 84. 

 These appear to constitute a complete series from the axis to the last cervical inclu- 

 sive. The atlas would then be the only cervical vertebra missing from this series. 

 So far as is known there is no atlas of Diplodocus in our collections. Marsh has pub- 

 lished figures of the atlas which are reproduced here in Figs. 4 and 5. He nowhere 



