38 



INTRODUCTION. 



so-called centra, the articulary balls of whicli are 

 transversely elliptical, and the neural arch is either 

 closed above {vertebrx imbricaiim clausse, accordmg to 

 the definition of Mertens), as in the Discoglossidx and 

 Pelobatidx, or notched between the zygapophyses so 

 as to expose the spinal cord between every two verte- 

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



Fm. 15. 







Vertebral columns of (a) Discoglossus jjictus and (b) Rana esculenta, 

 upper and lower views. 



the lateral ojjenings for the exit of the spinal nerves 

 being also of large size, the vei-tebral column forms an 

 open work above and on the sides. Neural spines are 

 absent or repi^esented 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- 

 bra are procoelous the eighth is biconcave; the ninth 

 being invariably biconvex. 



The first vertebra is devoid of transverse processes 



