419 



of the cervical vertebrae of the Ichthyosaurus, and announced the dis- 

 covery that the atlas and axis are firmly united and strengthened be- 

 low by an accessory articulating bone. In this paper he shows, that 

 the union of the two vertebrae is perfect at all periods of the animal's 

 growth, and apparently in all the species of the genus hitherto dis- 

 covered, having observed it in vertebrae varying in size from half an 

 inch to seven inches and a half in diameter. Externally there is a 

 strong line of demarcation between the two bones, but internally the 

 cancelli appear to pass from one to the other. The atlas, indepen- 

 dently of the union of the two vertebrae, is distinguished by the 

 form of the anterior cavity for the reception of the basilar process of 

 the occipital bone 5 by the outer margin being rounded instead of 

 sharp, and by the triangular facet on the inferior part of the circum- 

 ference for the reception of the accessory bone : the axis, indepen- 

 dently also of its union with the atlas, differs from the other vertebrae, by 

 the facet on the under surface for the reception of the accessory bone : 

 and the third vertebra is also distinguished fi'om the remaining bones 

 of the neck by a facet for the articulation of a very small accessory 

 bone. The intervertebral cavities of the 4th and 5th cervical verte- 

 brae, the author states, are less than in the vertebra of the dorsal and 

 caudal regions, and the anterior cavity is considerably smoother than 

 the posterior one of the same vertebrae. 



Sir Philip Egerton states that the spinal column does not, as de- 

 scribed by other authors, decrease in diameter from the middle dorsal 

 vertebra to the atlas, but that the minimum diameter is attained about 

 the fifth cervical vertebra, from which point to the occipital bone the 

 increase in size is very rapid, the atlas being fully one fifth more in 

 diameter than in the above-mentioned bone. 



In the former memoir Sir Philip Egerton described only one acces- 

 sory bone in the cervical region of the Ichthyosaurus ; but in this paper 

 he proves that there are three, and proposes to designate them by the 

 name of subvertebral wedge-bones. One of them is supplementary 

 to the atlantal socket, another is common to the atlas and axis, and 

 the third, which agrees in form with the second, but is much smaller, 

 articulates on the under surface of the third vertebra. 



The author, then, in conclusion, enlarges upon the admirable 

 adaptation of the structure of the Ichthyosaurus to the habits of the 

 animal. 



A communication was afterwards made " On the coal-fields on the 

 north-western coast of Cumberland, &c,, &c. : " by the Rev. Pro- 

 fessor Sedgwick, M.A., F.R.S., F.G.S., and Williamson Peile, Esq., 

 of Whitehaven, F.G.S. 



In a former paper* the authors described the range of the carboni- 

 ferous limestone from the neighbourhood of Kirkby Stephen to 

 Egremont; and showed that the formation admitted of tv/o divi- 

 sions : the lower representing the scar limestone of the Yorkshire 

 sections, the upper (also like the Yorkshire sections) exhibiting 



* Proceedings Geol. Soo., vol, ii. p. 198. 

 2f 2 



