10 FOSSIL REPTILIA OF THE LONDON CLAY. 



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

 to be more called for in the Plesiosanrus, 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 T. XI, 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 vertebrae 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 T. IX, 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, p, 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 liy in 

 fig. 2, T. IX. In some species it is double,* and beneath the atlas it becomes ' auto- 

 genous' or is developed as a separate element, ea, ex, of the subjoined Cut, fig. 8, in which 

 condition the part is found beneath the centrums of two or three of the anterior cervical 

 vertebras in the Ichthyosaurus.! 



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



pjp. g ing Vertebrata, and have accordingly received the special names, the 



one of ' atlas,' the other of ' epistropheus' or ' 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 



Atlas and Axis vertebra ^ ar o e globular head is sustained upon the shoulder-like processes 



of the Crocodile. of the ' atlas.' In the Crocodile, these vertebrae are concealed by 



the peculiarly prolonged angle of the lower jaw in the side view of the skeleton in T. XI, 



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 ofbirds." (See his admirable Memoir of June 14th, 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, pi. x, fig. 2. 



f This interesting discovery was communicated by its author, Sir Philip de M. Grey Egerton, Bart., F.G.S., 

 to the Geological Society of London, in 1836, and is published in the fifth volume of the second series of 

 their Transactions, p. 187, pi. 14. 



