190 Sir P. G. Egerton, Bart, on certain peculiarities in the 



bone forms a rugose articulating surface, (fig. 4. a.) extending backwards from the semilunar 

 socket until it meets at a considerable angle a smaller facet (fig. 4. d.), formed by the truncation 

 of the distal extremity of the bone. The former corresponds in size with the plane on the front 

 of the atlas, and articulates with it ; the latter abuts against the anterior extremity of the second 

 subvertebral bone. In order that the arrangement of this part of the skeleton may be fully un- 

 derstood it will be necessary to consider the nature of the connexion between the head and trunk 

 of the animal. In the several species of Ichthyosaurus, the proportion of the head to the total 

 length of the body varies considerably, but is in all very large. The articulation with the trunk is 

 effected by a ball and socket joint, between the basilar* portion of the occipital bone and the atlas. 

 How far the mechanism of this joint would have been complete without the aid of a supple- 

 mentary bone will be immediately perceived by fitting the atlas to its corresponding occipital bone. 

 By the application of this test I have invariably found the cup of the atlas incapable of receiving 

 more than two thirds of the occipital condyle. In well-preserved specimens this portion of the 

 articulating process is defined by a slight depression on its surface t (fig- 7. m.). This seeming 

 deficiency in the socket of the atlas is more than compensated for by the supplementary bone I have 

 above described ; for the angle at which it unites with the atlas (fig. 6.) is so contrived that 

 the semilunar socket on its proximal extremity does not coincide with the plane of the atlantal 

 socket, but projects forwards in such a manner as to embrace the whole lower circumference of 

 the occipital condyle. An universal joint is thus effected at the same time that the subvertebral 

 bone affords by its form, solidity, and position, a most powerful fulcrum to restrain the downward 

 pressure of the animal's head. 



Second Subvertebral Wedge Bone. 



This bone (Plate XIV. fig. 7. h.), like the one already described, is strengthened below by a 

 boss of solid bone. It is generally lozenge-shaped, but variations occur both in its form and 

 proportions, probably consequent upon the species and age of the animal to which it belonged. 

 Above, it is divided by a transverse elevation (fig. 3. b.) into two facets articulating with 

 those between the margins of the atlas and axis, and, in front, it impinges upon the extremity 

 of the first subvertebral bone, affording it a strong basis of attachment, and greatly enhancing its 

 power. 



Third Subvertebral Wedge Bone. 



This bone (Plate XIV. fig. 7. i.) resembles the second in form, but is considerably smaller. 

 It is lodged in a depression between the axis and the third cervical vertebra, and imparts 

 much firmness to these bones, at the same time that it does not materially affect the amount of 

 motion between them. I have before stated that the third subvertebral bone occurs rarely, and is 

 probably in some species altogether wanting. This may reasonably be inferred, if there be any 

 truth in the argument, by which 1 shall endeavour hereafter to show the office, which I believe 

 these bones were destined to perform. 



Cervical Vertebrae. 

 The third cervical vertebra of those species of Ichthyosaurus which are 



* This appears to have remained distinct from the other elements of the occipital bone in in- 

 dividuals of every age and species. Mr. Owen remarks that evidence is thus afforded of a 

 languid circulation in this family of Saurians. 



t See a gigantic vertebral column in the British Museum. 



