50 



BULLETIN 110, UNITED STATES NATIONAL MUSEUM. 



Whether they increase or decrease in length posteriorly can not be determined 

 from the present specimens, as all except the one described above lack their distal 

 extremities. 



The changes that take place in the cervical ribs from first to last are well shown 

 in figure 18. The rib of the sixth cervical was found articulated with it. The others, 

 though disarticulated, were not far removed from their respective vertebrae in the 

 matrix. 



The right rib of the eighth cervical, which may be considered typical of the 

 more posterior cervical ribs, as regards their general shape, is shown in figure 34. 



The tuberculum is not lengthened, being braced on the inner side by a thin 

 vertical plate of bone that rises from the top of the short capitular process. Ante- 

 riorly this rib is broad at the base, but narrows rapidly backward. It is thin 

 throughout; the anterior half, concave above, and convex below. The process 

 extending in front of the tuberculum is incomplete, but it was probably roundly 

 pointed. It extends more directly forward and not downward, as in the more 



anterior ribs. The 

 capitulum is short but 

 exceedingly heavy, 

 with a subcircular, 

 roughened articular 

 obtuse face that fits 

 closely into the deep 

 cup-like par apophysis 

 of the vertebra. 

 When articulated the 

 general direction of 

 the rib is backward 

 and downward with 



Fig. 34.— Cervical bib of the bight side of the eighth ceevical of Anteodemus the head in advance 



VALENS LEIDT. INNER ASPECT. NO. 4734, U.S.N.M. \ NAT. SIZE. C, CAPITULUM; p ,i tllhprole 



t, TUBEBCULUM. 



The rib of the ninth 

 cervical, No. 8367, U.S.N.M. (fig. 18), differs principally in its larger size and the 

 more rapid diminution of its posteriorly directed process. 



Thoracic ribs. — In Antrodemus there are fourteen ribs in the complete thoracic 

 series on each side. With specimen No. 8367, U.S.N.M., there are eleven ribs from 

 the right side, most of which are nearly complete, though all lack their extreme 

 distal ends. They appear to represent a continuous series from the first back to 

 the twentieth presacral. 



The changes that take place in their form from front to back are well shown 

 in figure 36. 



In figure 35 is shown the inner view of what I regard as the first dorsal rib, 

 though it might with almost equal propriety be considered as belonging to the 

 cervical series. Certainly it is transitional in character, and the distal end in all 

 probability was not attached to the sternum. The head of this rib takes on the 

 general form of the succeeding thoracic ribs in so much as the tuberculum and capi- 

 tulum are wide apart and lie in the same plane with articular surfaces of about equal 



