26 MEMOIRS OP THE CARNEGIE MUSEUM 



versely instead of concave in either direction as in the cervicals. Aside from 

 these there are no sudden changes in the transition from cervicals to dorsals. The 

 spines continue paired, the zygapophyses broadly expanded, while the centra are 

 long in the anterior dorsals but rapidly shorten posteriorly. All the vertebrse 

 between the last cervical and first sacral bear ribs, so that there are no lumbars 

 in Diplodocus. 



The First and Second Dorsals. — The superior branches of the anterior blades of the 

 horizontal laminoe in these vertebrae are considerably modified in order to give a 

 more substantial support for the suspension of the scapulae. They are broadly ex- 

 panded and inflected externally so as to present a broad, rugose external surface, 

 much more continuous and prominent than that just described as obtaining in C. 

 15. The prezygapophyses in these vertebrae differ from those in the former by be- 

 ing supported inferiorly by a pair of intersecting laminse. The paired spines are not 

 so broad and are more acutely terminated than in the cervicals, and there is a short 

 median spine. The pleuro-central cavities are deep and are not divided into two by 

 the pleuro-central laminae mentioned as present in the immediately preceding cervi- 

 cals. The inferior blade of the diapophysial lamina is obliquely directed down- 

 ward and backward, and opposes the modified superior branch of the anterior blade 

 of the horizontal lamina, while the posterior blade of the latter is directly opposed 

 to the inferior branch of its anterior blade. In this manner all these laminae oc- 

 cupy oblique positions, but meet at right angles and enclose the pre-, post-, supra-, 

 and infradiapophysial cavities. The latter in these vertebrae occupy their normal 

 positions, which has suggested the names applied to them in this paper. The ex- 

 tremities of the transverse processes are continued into downwardly extended dia- 

 pophyses terminating inferiorly in the tubercular rib facets. The capitular facets 

 are small prominences seen on the anterior, inferior and external margins of the 

 centra just behind the ball and below the anterior borders of the pleuro-central cav- 

 ities. The two lateral laminae projecting inferiorly and externally from the pos- 

 terior portions of the cervical centra are wanting in these and the succeeding dor- 

 sals, and the inferior surface of that portion of the centra in these vertebrae is 

 uniformly convex, there being no infracentral cavity. The superior arms of the X 

 formed by the intersection of the inner of the inferior blades of the pre- and post- 

 zygapophysial laminae are much longer than the inferior arms. The pleuro-central 

 cavity in D. 2 is the smallest of any in the presacral series in proportion to the size 

 of the centrum. These increase in size both anteriorly and posteriorly from D. 2. 

 There is an elongated pit placed vertically on the apices of the balls of these cen- 

 tra. The centra of these vertebrae rapidly become shorter, as will be seen by a refer- 



