166 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. III. 



The Sacrum, which is the key-stone of the pelvic arch that 

 sustains the weight of the body on the hinder extremities, is 

 formed in existing Saurians of two vertebrae, the bodies of 

 which are coalesced ; and the posterior extremity of the 

 distal vertebra instead of having a ball or convexity, as in all 

 the bones composing the anterior part of the spine, is 

 concave. The transverse processes are very strong, thick, and 

 broad. 



In the colossal Saurians of the Wealden and Oolite, the 

 Iguanodon, Hylseosaurus, Megalosaurus and Pelorosaurus, 

 (and I believe also in two other genera,) the sacrum is com- 

 posed of five or six vertebrae anchylosed into a solid bony 

 arch, as in Mammalia and Birds, with peculiar modifications 

 in the arrangement of the apophyses ; of which we shall have 

 to treat more particularly in the sequel. 



BICONVEX CAUDAL VERTEBRA (Lign. 37). The first caudal 

 vertebra in the existing species of Crocodilian reptiles is 

 remarkable on account of its double convexity, a peculiarity 

 that appears to have escaped the observation of Baron Cuvier, 

 and other anatomists. 



In 1835, the discovery of the remains of a fossil Crocodile 

 in the Wealden strata near Swanage, (in Case A,) led me to 

 institute a careful examination of the vertebral column of 

 a large Gavial, in the collection of my distinguished friend 

 Dr. Grant, of University College j for at that time there was 

 not an articulated skeleton of a crocodile in the Hunterian 

 Museum. I then observed that the body of the first caudal 



tail of an Ox, to show the relatively large medullary cavity; a fact, 

 of M T hich those who indulge in the luxury of Ox-tail soup may easily 

 satisfy themselves. 



LIGN. 36. SECTIONS OF CAUDAL VERTEBRAE OF THE Ox. 



(^ nat. size.) 

 Fig. 1. Longitudinal section. 



2. Transverse section. 

 a, b, Medullary cavity in the bodies of the vertebrae. 



