NATURAL SCIENCES OF PHILADELPHIA. l'.t 



at the anterior extremity, thus producing a larger provision of surface in thai 

 position for the articulation of the chevron. The neural arch in the Bpecimen 

 has apparently been so much laterally compressed, that its original condition 

 cannot be ascertained. 



It was upon several similar isolated vertebrae to the one just described thai tbi 

 genus Discosaurus was established, and I shall now proceed to examine them 

 comparatively with the object of determining their relationship with the Kan- 

 sas saurian. 



The genus Discosaurus was originally indicated from a mutilated body of a 

 caudal vertebra from the cretaceous formation of Alabama (Proc. Acad. Nat. 

 Sci. 1851, 326). In its imperfect condition, its peculiar character, and resem- 

 blance to a vertebra represented, by Prof. Owen, as a cervical of Plesiosaurut 

 pachyomus (British Fossil Reptiles, pi. 28,) it was mistaken for a cervical. 

 The specimen, together with another from the same individual, were described 

 in their true position, as relates to the regions of the vertebral column, in the 

 " Cretaceous Reptiles of the United States." They are represented in figs. 4, 

 5, 6, pi. v, of that work. The two vertebral bodies bear a near resemblance 

 with that of the Kansas saurian above described. They have nearly the 3am i 

 proportions, and are slightly smaller and rather less contracted intermediately. 

 The interchevron ridges are comparatively less well developed. The discoid 

 arrangement of the articular ends is even more conspicuous. The disks are 

 more prominent, more decidedly defined from the ledge beyond by a deeper 

 constriction or groove. The periphery of the disks is rather more convex, and 

 the surface towards the centre slightly less concave. The ontside ledge is 

 comparatively narrow and is not everted. The chevron articular surfaces ap- 

 pear rather to be produced through deflections of the groove defining the disk, 

 than by an extension of the ledge, so that the chevrons would appear to have 

 articulated with the depressed margin of the disk itself instead of with the 

 ledge. The neural arch is broken away in both specimens, except part of a i 

 abutment in one of them. 



The slight difference in size of these vertebrae from that of the Kansas 

 saurian, as well as the feebler production of the interchevron ridges, might 

 readily be due to their more posterior position in the column. The more de- 

 cided constriction of the disks at the articular extremeties from the rest of the 

 body, which led to the application of the name of Discosaurus, might be re- 

 garded as an individual peculiarity, or a variation in specific character; which 

 is often greater than real specific or even generic characters. 



A similar vertebra, from the lower cretaceous of Mississippi, represented in 

 figs. 10 12, pi. v of the " Cretaceous Reptiles of the United states,'' likewise 

 referred to Discosaurus, has the same size and proportions as that above de- 

 scribed of the Kansas saurian. In its anatomical characters it is intermediate- 

 to the latter, and those of the Alabama saurian. The interchevron ridges are 

 intermediate in degree of development. The most marked difference exists at 

 the articular ends of the body. The articular surface is comparatively flat, 

 being but feebly depressed towards the centre and as feebly rounded towards 

 the periphery, which extends to the acute margin defining it from the sides of 

 the body, almost without the intervention of a groove such as is described in 

 the preceding specimens. A slight deflection in the course of the acute 

 border defines the anterior chevron articulatioa. The posterior chevron ar- 

 ticulation is comparatively large and more distinctly defined than the corres- 

 ponding one in the Kansas and Alabama specimens. The sides of the neural 

 arch ascend in a steep slope from the more concave sides of the body, sweep- 

 ing outwardly to the upper border of the costal pit. The diameter of the 

 spinal canal is about seven lines. 



Another vertebral body, from the cretaceous formation of New Jersey, re- 

 ferred to Discosaurus, is represented in figs. 79 of the work above indicated. 

 It presents characters in common with the preceding specimens, but likewise 

 has peculiarities of a decided kind. It approaches most the Alabama speci- 



1870.] 



