20 MEMOIES OF THE CARNEGIE MUSEUM 



ward. There are prominent rugosities just above the postzygapoj)hyses. There is 

 a prominent transverse process springing from the middle of the sides of the neural 

 arch. It is broad and thin and is directed downward, backward and outward, 

 and terminates inferiorly in a small spatulate expansion. The posterior blade of 

 the horizontal lamina extends from the transverse process to the posterior zygapo- 

 physis at an ascending angle of 45°. The inferior blade of the diapophysial lamina 

 has a horizontal position and supports the posteriorly projected transverse process 

 by forming a short laminar buttress connecting that process with the sides of the 

 neural arch and separating the infradiapophysial cavity from the postdiapophysial 

 cavity. This latter cavity is separated from the supradiapophysial cavity by the 

 posterior blade of the horizontal lamina. There is no prediapophysial cavity in the 

 axis. A short cervical rib without anterior process springs from the side of the 

 centrum near its inferior margin and anterior extremity. Only the base of the 

 odontoid process is preserved, but this indicates that it was of moderate length, with 

 a slightly concave superior surface. The centrum is strongly opisthocoelous, as is 

 the case also in all the succeeding cervicals and anterior dorsals. The central artic- 

 ulations of the cervicals and anterior dorsals of Diplodocus are the most finished of 

 all the articular surfaces in the entire skeleton. There are deep pleuro-central 

 cavities which extend anteriorly into the base of the odontoid process. Posteriorly 

 these cavities are only separated from the cup for the ball of the succeeding vertebra 



by a thin plate of bone. Thus the body of the centrum is 

 practically destroyed, and instead of consisting of a solid bony 

 cylinder it is reduced to four thin plates. These unite to form 

 the median longitudinal axis of the centrum. From this 

 axis these plates diverge at right angles and partially enclose 

 the neural canal and the infra- and pleuro-central cavities. 

 At their anterior extremities they are united by a convex disc 

 of bone, the ball or odontoid process ; while at their posterior 

 Fig. 6. Axis of Dip- extremities they are united by the concave disc of bone which 

 lodocus carnegit; seen from fomis the cup for the reception of the ball of the succeeding 



right side, one fifth natural . , ttt-,! , • • ,• .^ • , , -i 



„ ^. o. ^ cervical. With certain variations this structure prevails 



size. From No. 84, Car- _ '• 



neo-ie Museum collections. throughout the cciitra of all the cervicals. In this manner 



the inferior border of the cervical centra are concave both 

 transversely and longitudinally instead of convex transversely and concave longi- 

 tudinally as in the dorsals. The principal characters of the axis are well shown 

 in Fig. 6. 



Cervicals Three, Four, and Five. — All of these vertebrae are more or less injured. 



