86 BRITISH FOSSIL REPTILES. 



retained in both segments, as in all the other vertebrte of tlie trunk. That of the 

 atlas, fig. 8, pi. a, is a simple slender style, articulated by the head only, to the 

 independently developed inferior part of the centrum, or ' hypapopliysis' (ra, ex). The 

 neurapophyses {no) of the atlas retain their primitive distinctness ; each rests in part 

 upon the proper body of the atlas (ca), in part upon the hypapophysis. The neural 

 spine {ns, a) is also here an independent part, and rests upon the upper extremities 

 of the neurapophyses. It is broad and flat, and prepares us for the farther 

 metamorphosis of the corresponding element in the cranial vertebrae. 



The centrum of the atlas {ca), called the odontoid process of the axis in Human 

 Anatomy, here supports the abnormally-advanced rib of the axis vertebra, which in 

 some Crocodilia is articulated by a bifurcate extremity, like the ribs of the succeeding 

 cervical vertebrae ; but it is not expanded or hatchet-shaped at the free extremity. 

 The proper centrum of the axis vertebra {c,r) is the only one in the cervical series 

 which does not support a rib ; it articulates by suture with its neurapophyses {nx), and 

 is characterised by having its anterior surface flat, and its posterior one convex. 



With the exception of the two sacral vertebrae, the bodies of which have one articular 

 surface flat and the otlier concave, and of the first caudal vertebra, the body of which has 

 both articular surfaces convex, the bodies of all the vertebrse beyond the axis have the 

 anterior articular surface concave, and the posterior one convex, and articulate with one 

 another by ball-and-socket joints. This type of vertebra, which I have termed ' procoe- 

 lian,'* characterises all the existing genera and species of the family Crocodilia, with all 

 the extinct species of the tertiary periods, and also two extinct species of the Greensand 

 formation in New Jersey. f Here, so far as our present knowledge extends, the type 

 was lost, and other dispositions of the articular surfaces of the centrum occur in the 

 vertebrae of the Crocodilia of the older secondary formations. The only known 

 Crocodihan genus of the periods antecedent to the Chalk and Greensand deposits with 

 vertebrae articulated together by ball-and-socket joints, have the position of the cup 

 and the ball the reverse of that in the modern Crocodiles, and the genus, thus cha- 

 racterised by vertebrae of the ' opisthococlian' type, has accordingly been termed 

 Streptospotidi/hi.s, signifying ' vertebrae reversed.' The aspects of the zygapophyses 

 are, however, more constant ; the anterior ones, PI. 1 D, fig. 3 ~, look obliquely inwards ; 

 the posterior ones, ib. ~', obliquely outwards. In a vertical section, therefore, of a 

 Crocodilian vertebra, such as is figured in PI. 4, fig. 3, the smooth, flattened inner 

 surface of the anterior zygapophysis is turned towards the observer, and the convex 

 outer surface of the posterior zygapophysis. Thus the anterior and posterior extremity 

 of the vei'tebra being determined by observation of the aspect and direction of the 

 zygapophyses, it is at once seen whether the body has the procoelian structure, as in 

 PI. 4, fig. 3, or the opisthococlian structure, as in fig. 4. But the most prevalent type 



* npos, before ; koiXos, concave. 



f . Quarterly Journal of the Geological Society, November 1849. 



