CROCODILIA. 85 



The purport of this modification is the same in the Crocodilia as that which seems 

 to be more called for in the Plesiosaurus, viz. to augment the strength of the cervical 

 region of the skeleton ; and this is so effectually done by the overlapping of the hatchet- 

 shaped ribs of this region in the Crocodilia, as shown in Plate 1 , that the flexibility of 

 the neck is much restricted, although the joint of the head allows that part to be 

 bent from side to side at nearly right angles with the neck. When, however, the head 

 is held firmly forwards by its powerful muscles, the imbricated vertebrse of the neck 

 transmit with great effect the impulse which the strong and long tail gives to the rest 

 of the body in the act of swimming. 



In fig. 3 the cervical vertebra is represented minus its pleurapophyses, and it 

 answers accordingly to that portion of the natural segment to which the term ' vertebra' 

 is usually restricted in the dorsal region of the trunk. The exogenous processes shown 

 in this view of the vertebra are, ^j, the ' parapophysis' or inferior transverse process, 

 developed from the centrum ; d, the ' diapophysis' or upper transverse process de- 

 veloped, as in most cases it is, from the neurapophysis ; z, z, are the ' zygapophyses' 

 or ' oblique processes,' which, from their function in articulating together contiguous 

 vertebrae, are also called ' articular processes.' In most of the cervical, and in some 

 of the dorsal, vertebrae of the Crocodile, an exogenous process is developed from the 

 under surface of the centrum, called ' hypapophysis ;' it is indicated by the letters hy in 

 fig. 2. In some species it is double,* and beneath the atlas it becomes ' autogenous' or is 

 developed as a separate element, ca, ex, fig. 8, in which condition the part is found beneath 

 the centrums of two or three of the anterior cervical vertebras in the Ichthyosaurus. f 



The first and second vertebrae of the neck are peculiarly modified in most air- 

 breathing Vertebrata, and have accordingly received the special names, the one of 

 ' atlas,' the other of ' axis.' In Comparative Anatomy these become arbitrary terms, 

 the properties being soon lost which suggested those names to the 

 human anatomist ; the ' atlas' e. g. has no power of rotation upon 

 the ' axis' in the Crocodile, and it is only in the upright skeleton 

 of man that the large globular head is sustained upon the shoulder- 

 hke processes of the ' atlas.' In the Crocodile, these vertebrae 

 are concealed by the pecvdiarly prolonged angle of the lower jaw 

 in the side view of the skeleton in Plate 1 , and a woodcut of the , ,, , , • , , 



' Atlas and Axis vertebrie 



two vertebrae is therefore subjoined. The pleurapophyses are of the Crocodile. 



previously extended the same homology to the "particularly prominent wing-like appendages to the 

 transverse processes in many of the long-necked quadrupeds, and the long styloid processes of the cervical 

 vertebra of birds." (See his admirable Memoir of June 1 Ith, 1822, in the Geol. Trans., 2d series, vol. i, p. 384.) 



* In Crocodilus basifissus, e. g., see the Quarterly Journal of the Geological Society, November 1849, 

 p.381,pl. X, fig. 2. 



t This interesting discovery was communicated by its author, Sir Phihp de M. Grey Egerton, Bart., to 

 the Geological Society of London, in 18.36, and is published in the fifth volume of the second series of their 

 Transactions, p. 187, pi. 14. 



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