38 



INTRODUCTION. 



so-called centra, the artioulary balls of which are 

 transversely elliptical, and the neural arch is either 

 closed above (vertebras imbricatim clauses, according to 

 the definition of Mertens), as in the Discoglossidas and 

 Pelobatidas, or notched between the zygapophyses so 

 as to expose the spinal cord between every two verte- 

 bras ; the latter type is most marked in Bana, in which, 



Fig. 15. 



c^*S>> 



l 



r? 



B 



Vertebral columns of (a) Discoglossus pictus and (b) Rana eseulenta, 



upper and lower views. 



the lateral openings for the exit of the spinal nerves 

 being also of large size, the vertebral column forms an 

 open work above and on the sides. Neural spines are 

 absent or represented by a low keel, which is much 

 prolonged posteriorly in Discoglossus and Pelobates. 

 The articulation is convexo-concave or opisthoccelous 

 in the Discoglossidas, concavo-convex or procoelous in 

 the other groups. In those forms in which the verte- 

 bras are procoelous the eighth is biconcave; the ninth 

 being invariably biconvex. 



The first vertebra is devoid of transverse processes 



