GENERAL STRUCTURE. 



IO49 



Fig. 976. — Profile and lateral 

 views of the crown of a tooth of a 

 Dinosaur (Massospondylus), with 

 the marginal serrations magnified ; 

 from the Mesozoic of India. 



When both pairs of limbs are present the vertebral column 

 is differentiated into cervical, dorsal, lumbar, sacral, and caudal 

 regions ; and the vertebrae themselves are 

 invariably ossified, although in some prim- 

 itive types a small notochordal canal may 

 perforate their centra. In a large number 

 of existing forms the majority of the centra 

 are proccelous ; but they are amphiccelous 

 in many fossil and a few existing types ; 

 while among the Dinosauria and Chelonia 

 an opisthoccelous type is common in parts 

 of the series. In all cases there is an in- 

 tercentrum between the skull and the atlas ; 

 which may either form the inferior ring of 



the latter, or, when the centrum of the atlas is separate (Ichthyoptery- 

 gia), may be of the normal wedge-like form. In some groups addi- 

 tional articulations may be developed on the arches of the trunk verte- 

 bras, taking the form of a wedge-shaped process, or zygosphene, fitting 

 into a corresponding cavity, or zygantrum (fig. 977). The transverse 

 processes of the dorsal verte- 

 brae may be either long (fig. 

 1058) or very short (fig. 

 977); and the ribs may ar- 

 ticulate either by a single 

 head with the transverse 

 process, or by two heads 

 to different portions of the 

 same process, or by one ar- 

 ticulation to the latter and 

 by another to a facet on the arch or the centrum ; there being great 

 variation as to the position of the transverse processes and rib-facets 

 in different groups, and also in the different parts of the column of 

 a single animal. Occasionally the ribs articulate at the junction of 

 two vertebrae. Chevron-bones are generally present in the tail ; and 

 intercentra may be retained. No living Reptile with limbs has less 

 than two sacral vertebrae, and in certain extinct forms the number 

 may be increased to five or six. In nearly all Reptiles the tail is 

 well developed. The ribs may have uncinate processes. In many 

 existing forms the sternum, which may be ossified, is rhomboidal, 

 and may have the last pair of ribs attached to a backward median 

 process. Its structure in many fossil groups is not known, but 

 according to Professor Marsh's interpretation some of the Dino- 

 saurs had paired sternal ossifications, corresponding to the two 

 centres from which the sternum develops in the Ratite Birds. 

 Abdominal ribs may be developed in the parietes of the ventral 



B 



Fig. 977. — Posterior (a) and haemal (b) aspects of a 

 dorsal vertebra of a Snake. The cavities in A on the 

 sides of the neural canal are the zygantra. 



