24 EXPLANATION OF PLATES U. 12. 



beneath, exhibiting the course of its over lapping 

 bones. (Conybeare.) 



A. Tooth of a Crocodile, shewing 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 shewn 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. Vol. 1. p. 181. 



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 Ichthyosau- 



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



* Sir Philip de Malpas Grey Egerton has pointed out some beau- 

 tiful examples, hitherto unnoticed, In the Atlas and cervical Ver- 

 tebrae of Ichthyosauri, of peculiar mechanical contrivances to sup- 

 port and regulate 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 sur- 

 faces, admitting of the least flexure of any of the Vertebras 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. 



