24 EXPLANATION OF PLATES 11. 12. 



beneath, exhibiting the course of its over-lapping 

 bones. (Conybeare.) 



A. Tooth of a Crocodile, showing the incipient absorp- 



tion of the hollow cone which forms its base, from 

 the effect of pressure of a new tooth rising beneath. 

 (Conybeare.) 



B. Similar effects shown in the transverse section of the 



upper and lower jaws of an Ichthyosaurus. (Cuvier.) 



C. Example of the same kind of absorption produced by 



the pressure of a new tooth, on the base of an older 

 tooth in the jaw of Ichthyosaurus. (Conybeare.) 



Plate 12. V. I. p. 142. 



1. Sternal Arch and Paddles of Ichthyosaurus. See V. I. 



p. 182, Note. (Home.) 



2. Sternal Arch of Ornithorhynchus. (Home.) 



3. 4, 5, 6. Occipital and Cervical Bones of Ichthyosaurus, 



from the Lias at Lyme Regis.* (Original.) 



* Sir Philip de Malpas Grey Egerton has pointed out some beautiful 

 examples, hitherto unnoticed, in the Atlas and cervical Vertebrae of 

 Ichthyosauri, of peculiar mechanical contrivances to support and regu- 

 late the movements of their enormous heads. (See Lond. and Edin. 

 Phil. Mag. Nov. 1835. p. 414.) 



Fig. 3, a. represents the Basilar portion of the Occipital bone of a 

 very large and aged Ichthyosaurus, from the Lias of Lyme Regis, 

 (scale one-eighth.) The nearly hemispherical process (a) articulated 

 with a comparatively shallow socket in front of the Atlas, (4. a.) 

 and this ball and socket, or universal joint, gave freedom of motion and 

 support to a weighty head. 



Fig. 4. Atlas and Axis of a very young Ichthyosaurus, (two-thirds of 

 nat. size.) These bones adhere together by two nearly flat surfaces, 

 admitting of the least flexure of any of the Vertebrae in the whole 

 body, but giving the greatest strength to that part of the Column, 

 where strength rather than flexure was required. 



On the inferior margins of the Atlas and Axis and third cervical 

 vertebra, are triangular facets articulating with three strong wedge- 

 shaped sub-vertebral bones (c) hitherto undescribed. 



