PKOF. OWEN ON SOME DINOSAURIAN VERTEBRA. 43 



3. On a new Modification of Dinosaurian Vertebra. By Prof. 

 Owen, C.B., F.R.S., F.G.S., &c. (Read November 17, 1875.) 



[Plates IY. & V.] 



In Dinosauria, after the character of " number of coalesced vertebras 

 with alternate disposition of their centrums and neural arches in the 

 sacrum," that of " the complex development of the neural arch with 

 par- and diapophyses in antecedent trunk-vertebras " prevails ; the 

 modifications of the articular surfaces of the centrums are less 

 constant. 



Most commonly those surfaces, through the major part of the 

 spinal column, are more or less flattened, the deviation from flat- 

 ness being usually toward concavity on the hinder surface. 



In this modification it is usual to find a few vertebras at the fore 

 part of the column with such concavity deepened to a cup and fitted 

 to a ball on the fore end of the next centrum : such modification is 

 termed " opisthoccelian ;" but I have not seen any example of a Dino- 

 saur in which it prevailed throughout the dorsal region. In certain 

 caudal vertebras the deviation from flatness is toward concavity on 

 both articular surfaces of the centrum ; but I have not met with 

 any instance in which the depth was such (as in the Ichthyosaurus 

 e.g.) as to merit the term " amphiccelous ;" and I know of no Dinosaur 

 from the Lias upwards in which the dorsal vertebras are so modified. 

 The nearest approach is made by the vertebras of the phytophagous 

 Dinosaurian genera Tapinocephalus and Pareiasaurus ; but it is so 

 far from the true, or commonly understood, amphiccelian character, 

 that I venture to hope the Society will hold me justified in proposing 

 another term for such vertebral modification. 



Of the dorsal series of vertebras in TapinocepJialus Atherstonii, Ow., 

 the subject of PI. IV. is a type. The centrum, 2| inches in length, 

 is 5 inches in breadth, and 4| inches in height. Both articular sur- 

 faces deviate from flatness by a feeble concavity, a little more marked 

 on the hinder one. 



As at first wrought out of the matrix, which the petrified bone 

 closely resembles in colour, no deviation from this common Dinosau- 

 rian character presented itself; and when, persisting in chiseling 

 away whatever layer was doubtful, the defined central depression (fig. 

 1, c) came into view, a doubt whether it was natural at first suggested 

 itself. Similar thoroughness in exposing the veritable bony surface 

 in other vertebras, however, was attended with the same result. The 

 small, central, subcircular pit, from 4 to 5 (rarely 6) lines in diameter, 

 was repeated in each case. On the free surface of the anterior 

 sacral, as on that of the posterior of four anchylosed sacrals of the 

 Dinosaurian type, in Tapinocephalus, the foramen was relatively 

 larger than in the dorso-lumbar series. The neural arch of the 

 above fourth sacrals had been severed by fracture from an anchy- 

 losed fifth sacral ; but the intervertebral space of the centrums had 

 not been obliterated. 



