288 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 6, 



Fi?*st sacral vertebra. in, tenths. 



Centrum. — Vertically 1 3 



Transversely 1 65 



Upper edge of centrum to summit of neural spine 2 2 



Neural spine — Vertically 1 55 



Longitudinally (about) 7 



Lower edge of body to origin of transverse process 75 



Thickness of transverse process 82 



Length 25 



Sacral rib. — Length 1 3 



Thickness of distal end 6 



Caudal region.— Total 26 



Eleventh caudal vertebra. 



Centrum. — Length 9 



Vertical 1 15 



Transverse 1 35 



From upper edge of centrum to summit of neural spine 2 8 



Length of neural spine. — Vertically (about) 1 1 



Longitudinally 65 



Ribs. — 11th measured along its curve 11 



Diameter of head 7 



Diameter of body 45 



Humerus. — Long 7 2 



Thickness of anterior end from above downwards ... 1 9 



Expanded distal end -I .i.^L ft 5 



Femur. — Long 7 15 



Anterior extremity from above downwards 1 8 



The Structure of the Atlas and Axis. 



Thanks to the investigations of Sir Philip Egerton, the structure 

 of the axis and atlas of Ichthyosaurus is placed beyond doubt. But 

 our knowledge of the corresponding parts of Plesiosaurus cannot be 

 said to be by any means so well based, since it rests, so far as I am 

 aware, upon the examination of a single and imperfect specimen, 

 which has been described in the following terms by Professor Owen : — 



" A recent opportunity of examining the atlas and axis of the Ple- 

 siosaurus, kindly afforded me by my friend Professor Sedgwick, has 

 not only strengthened this view of the general nature of the ' subver- 

 tebral wedge-bones/ but has made me incline to the second hypothesis 

 of the special homology of the first or anterior of the wedge-bones 

 which is proposed in my ' Report on British Fossil Reptiles,' viz. : — 

 That it answered to the part described as the body of the atlas, in 

 the existing Saurians and Chelonians ; which therefore may be re- 

 garded, like the first subvertebral wedge-bone, as the cortical part 

 only of such vertebral body, like the plate of bone beneath the bi- 

 concave central part of the body of the atlas in the Siluroid fish. 



" The atlas and axis in the Plesiosaurus (fig. 3) preserve the general 

 proportions of the other cervical vertebrae, and are consequently longer 

 than their homologues in the Ichthyosaurus ; but they are similarly 

 anchylosed together, and measure A\ centimetres (nearly 2 inches) 

 in length, 3 centimetres across the anterior concave surface of the 

 atlas, and 3£ centimetres across the less concave posterior surface : 



