112 REPORT 1841. 



part of the body, with the costal surface directed obliquely outwards and for- 

 wards. There is a small costal surface at the side of the expanded posterior 

 extremity of the same vertebra, against which a part of the head of the fourth 

 rib abuts ; that and three of the succeeding ribs having their heads applied 

 over the interspace of two contiguous vertebrae, as nearly throughout the tho- 

 racic region in Mammalia. The lateral compression of the centrum increases 

 in the sixth and seventh (conspicuous) vertebrae, in which the under surface 

 forms an obtuse ridge ; in the eighth vertebra this surface is broader. In 

 none of these vertebrae is a process developed from the under surface as in the 

 hinder cervical and anterior dorsal vertebrae of the Crocodiles. 



The most striking character of the vertebrae of the Hylaosaurus is the 

 great development of the neural arch and its processes. The anterior articu- 

 lar processes extend (in the anterior dorsal and cervical vertebra) over half 

 the centrum next in front, and a broad upper transverse process is developed 

 from the side of the neurapophysis and along its anterior continuation : this 

 transverse process extends horizontally outwards, is notched anteriorly, and 

 contracts to an obtuse point against which the tubercle of the rib articulates : 

 the transverse processes are flat transversely, slightly concave lengthwise, and 

 smooth below : they increase in length and strength as the vertebrae extend 

 along the trunk ; and the ribs, which they contribute to support, exhibit a still 

 more rapid increase : the ribs present, as in the other Dinosaurs and Croco- 

 diles, a bifurcated vertebral end for the double articulation above described*. 

 The neck and head of the rib corresponding with the seventh conspicuous ver- 

 tebra is 2 inches 2 lines in length ; the tubercle, or upper head, is 10 lines long ; 

 the breadth of the rib at the point of bifurcation is 1 inch 1 line. The neck 

 of the eighth rib has the same length as that of the seventh, but is twice as 

 thick and strong ; the tubercle is broader but shorter. Beyond the tubercle 

 the shaft of the rib is bent at right angles with the neck. This soon begins to 

 shorten, and the shaft of the rib to lengthen, until it becomes attached solely 

 to the transverse process. 



In the dorsal vertebrae the body increases in all its proportions, excepting 

 its length. The lateral compression now manifests itself at the upper part 

 of the centrum just below the neurapophysial suture ; the under surface of 

 the posterior dorsal and lumbar vertebra is convex transversely, but in a less 

 degree than in the Megalosaurus, and in some, it is obscurely carinated. The 

 external surface at the middle contracted part of the vertebra is moderately 

 smooth, but the minute striee give it a somewhat silky lustre ; it is longitu- 

 dinally but irregularly ridged and grooved near the articular ends. These 

 are both slightly concave at the centre, more slightly convex near the circum- 

 ference. The difference between the vertebrae of the Hylaeosaur and the bi- 

 concave Crocodilian vertebrae is chiefly manifested in the development of 

 the neural arch. The modification of this part in the cervical vertebrae has 

 already been mentioned. In the dorsal vertebrae each neurapophysis rises 

 vertically, contracting in the axis of the vertebra, expanding transversely or 

 outwardly, until it has attained a height equal to that of the centrum ; there it 

 expands into a broad and flat platform, from the middle line of which the 

 broad spine is developed. A vertically compressed but sti'ong transverse pro- 



* Dr. Mantell, io his Memoir on the Tilgate Saurians, Philos. Transactions, Part ii. for 1841, 

 which I received while this Report was going through the press, says that " the hilobed head 

 and the great external expansion of the arch of the rib in all probability bears a relation to 

 the enormous development of the dermal spines," p. 143. But this is precisely the modifica- 

 tion of the skeleton in which the Hylaosaurus differs most from the existing Saurians which 

 possess such spines, as the Cyclura, and in which it most resembles the Iguanodon and Me- 

 galosaurus. 



