12 FOSSIL REPTILIA OF THE LONDON CLAY. 



proccelian structure, as in the true Crocodiles, T. IV, T. IX, or the opisthocoelian 

 structure, as in the Slreptospondylus. But the most prevalent type of vertebra amongst 

 the Crocodilia of the secondary periods was that in which both articular surfaces of 

 the centrum were concave, but in a less degree than in the single concave surface of 

 the vertebrae united by ball and socket. A section of a vertebra of this ' amphicoelian' 

 type, such as existed in the Teleosaurus and Sleneosaurus, will be figured in a subsequent 

 Monograph. In the Ichthyosaurus, the concave surfaces are usually remarkable for 

 their depth, the vertebrae resembling in this respect those of fishes. Some of the most 

 gigantic of the Crocodilia of the secondary strata had one end of the vertebral centrum 

 flattened, and the other (hinder) end concave ; this ' platyccelian' type we find in the 

 dorsal and caudal vertebrae of the gigantic Cetiosaurus. 



With a few exceptions, all the modern Reptiles of the order Lacertilia have the same 

 proccelian type of vertebras as the modern Crocodilia, and the same structure prevailed 

 as far back as the period of the Mosasaurus, and in some smaller members of the 

 Lacertilian order in the Cretaceous and Wealden epochs. 



Resuming the special description of the osteology of the modern Crocodilia, we find 

 the proccelian type of centrum established in the third cervical, which is shorter but 

 broader than the second ; a parapophysis is developed from the side of the centrum, 

 and a diapophysis from the base of the neural arch ; the pleurapophysis is shorter, its 

 fixed extremity is bifid, articulating to the two above-named processes ; its free ex- 

 tremity expands, and its anterior angle is directed forwards to abut against the inner 

 surface of the extremity of the rib of both the axis and atlas, whilst its posterior pro- 

 longation overlaps the rib of the fourth vertebra. 



The same general characters and imbricated coadaptation of the ribs characterise 

 the succeeding cervical vertebrae to the seventh inclusive, the hypapophysis (hy, fig. 2, 

 T. IX) progressively though slightly increasing in size. In the eighth cervical the 

 rib becomes elongated and slender ; the anterior angle is almost or quite suppressed, 

 and the posterior one more developed and produced more downwards, so as to form 

 the body of the rib, which terminates, however, in a free point. In the ninth 

 cervical the rib is increased in length, but is still what would be termed a ' false' 

 or ' floating rib' in anthropotomy. 



In the succeeding vertebra the pleurapophysis articulates with a haemapophysis, and 

 the haemal arch is completed by a haemal spine ; and by this completion of the typical 

 segment we distinguish the commencement of the series of dorsal vertebrae. With regard 

 to the so-called ' perforation of the transverse process,' this equally exists in the present 

 vertebra, as in the cervicals, as may be seen by comparing fig. 6, p. 5, Part I of this 

 Monograph, with fig. 2, T. IX, Part II ; in both, the foramen is the vacuity intercepted 

 between the bifurcate extremity of the rib and the rest of the vertebra with which 

 that rib articulates ; and, on the other hand, the cervical vertebrae equally show 

 surfaces for the articulation of ribs. Cuvier, in including the proximal portions 

 of the ribs with the rest of the vertebra, in his figure of a dorsal vertebra of a 



