500 Geological Society. 



mineral matter was supplied in the first instance from that of the 

 two last. 



The paper notices also the fossil wood from Lough Neagh and 

 Bonn, in which some small parts preserve their texture, although re- 

 maining still unchanged in the midst of the petrified mass. 



The author concludes with a short notice of the different conditions 

 in which the structure of wood is prestrved in different specimens, 

 and considers that the condition of the wood has not any influence on 

 the process of petrifaction. 



A paper was next read, entitled, " Further notice on certain pecu- 

 liarities of Structure in the Cervical Region of the Ichthyosaurus," 

 by Sir Philip Grey Egerton, Bart., M.P., V.P.G.S. 



In a former communication* Sir Philip Egerton gave an account 

 of the cervical vertebr* 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 ; 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 di.",tinguished from the remaining bones 

 of the neck by a facet fur 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 s;)inal 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 the above-mentioned bone. 



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

 sory bone in tiie 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 



• Lend, and Edinb. Phil. Ma^., vol. vii. p. 414. 



