CRETACEOUS FORMATIONS. 



17 



In the dorsal region, where the rib is supported wholly by a diapophysis 

 developed from the platform of the neural arch ( n p), the centrum has assumed the 

 ordinary subcircular shape, at least at its articular ends (Tab. VIII, fig. 3). The 

 surface is very slightly and uniformly concave in most, with a slight central 

 depression occupying about one third of the vertical diameter of the surface; but 

 in some, as in fig. 3, there is hardly any trace of the median depression. The 

 sides of the centrum are rather more concave lengthwise than in the cervical 

 series, but least so at the lower part. 



The following are dimensions of the dorsal vertebra : 



Length of centrum . 



Breadth of ditto, at articular end 

 Breadth of ditto, at the middle 

 Height of ditto, at articular end 

 Vertical diameter of outlet of neural canal 



Plesiosaurus latissimus. 

 In. lines. 

 6 



2 

 2 

 2 

 2 

 



11 



4 



10 



10 



The following are admeasurements of a dorsal vertebra, having a greater 

 proportion of the neural arch preserved : 



In. 

 Length of centrum . . . . . . . . . .2 



Depth of terminal surface ........ 2 



Breadth of ditto .......... 3 



Breadth of the middle of the centrum ...... 2 



From the under part of the centrum to the upper part of the 



diapophysis .......... 4 



From ditto to summit of neural spine .8 



Fore-and-aft extent of neural spine 2 



lines. 



10 



The chief changes observed in the middle dorsal vertebrae are the almost 

 circular contour of the articular ends of the centrum, and the minor antero- 

 posterior breadth of the neural spine. 



Of one of the dorsal ribs an extent of fourteen inches in length is preserved ; it 

 shows two flexures ; the first and shortest is concave upward, the rest convex 

 upward and outward, for half the extent of the rib, the rest being straight. Many 

 smaller parts of the ribs are scattered about the block of matrix. 



The coracoids exhibit the proportional size, and broad expanse, characteristic 

 of the genus; they are in too fractured and mutilated a state to serve for deter- 

 mination of any specific characters. One of the largest portions is figured in 

 Tab. IX, fig. 2. 



3 



