THE RIBS AND STERNUM. 133 



The Posterior Ribs. (Plate XXXIX, figures 4-9.) 



The posterior ribs gradually diminish in length, and become more 

 slender, but the curvature is still sti-ongly marked. In most of them, the 

 head is large, and the two articular faces on it, distinct. The tubercle 

 diminishes in size, and toward the last is a mere sessile articular tiice. 



The sternal riljs are not preserved in any of the known specimens of 

 the Diiiocf'rata, and they were probabl}^ unossified. TJieir general form 

 and position are indicated in the restorations. Plates LV and LVI. 



The more important dimensions of two postei'ior ribs in the type of 

 Dinoceras mirahUe are the following : 



3feast(reiiieiifs of Dorsal Rib. {Dinoceras rnirahile. No. 1036.) 



m. 



Totnl length of rib, in st raight line, .530 



Greatest diameter of head, .. . - .-. .- .052 



Transverse diameter of head, .037 



Least diameter of shaft, near the middle, .015 



Diameters of shaft, at broadest point, .- — 0-25-.060 



Measurements of Posterior Rib. {Dinoceras niirahile, No. 1036.) 



m. 



Total length of rib, in straight line, __ .520 



Greatest diameter of head, — -. .056 



Transverse diameter of head, -- .062 



Diameters of shaft, at broadest point, .016-051 



The Sternum. 



(Plate XL; and woodcut 133, below.) 



Sternal bones are preserved in a number of individuals of the 

 Dinmerata in the Yale Museum, but the entire series in any one 

 individual has not been recovered. The general form and character of 

 these bones are' shown in the figures, Plate XL. A series in the natural 

 position is represented in the woodcut below. 



