318 Geological Society, 



hinder extremities as well developed as in the generality of Sau- 

 rians. Of tliese extremities, as well as of the anterior and of the 

 head, there are no traces. 



Mr. Owen then pi-oceeds to determine to which division of Sau- 

 rians, having ball and socket vertebral joints, the fossil should be 

 referred. In the Crocodilian or Loricate group, the transverse costi- 

 gerous processes are elongated, and three, four, or live of the verte- 

 briB which precede the sacrum are ribless, and consequently reck- 

 oned as lumbar vertebrae : in the Lacertian Sauriae there are never 

 more than two lumbar vertebrae, and those which have ribs support 

 them on short convex processes or tubercles. 



In the fossil from the chalk, the ribs are articulated with short 

 processes of the kind just mentioned, resembling tubercles, and they 

 are attached to the sides of the anterior part of all the vertebrae, 

 except the one immediately preceding the sacrum. These charac- 

 ters, Mr. Owen says, in conjunction with the slenderness and uni- 

 form length of the ribs, and the degree of convexity in the articular 

 ball of the vertebree, prove incontestably, that the fossil is part of a 

 Saurian, appertaining to the inferior or Lacertian group. 



The under surface of the vertebrae is smooth, concave in the axis 

 of the spine, and convex transversely. As there are twenty-one 

 costal vertebrae anterior to the sacrum, including the single lumbar, 

 the fossil, Mr. Owen observes, cannot be referred to the genera 

 Stellio, Leiolepis, Basiliscus, Agama, Lyriocejjhalus, Anolis, or Cha- 

 mceleo7i, but that a comparison may be instituted between it and the 

 3Ionitors, Iguanas, and Scinks, In conclusion, he states, that in the 

 absence of the cranium, teeth, and extremities, any further approxi- 

 mation of the fossil would be hazardous, and too conjectural to yield 

 any good scientific result. 



May 13, 1840. — A memoir was commenced " On the Classification 

 and Distribution of the Older or Palaeozoic Rocks of the North of Ger- 

 many and of Belgium, as compared with formations of the same age 

 in the British Isles ;" by the Rev. Prof. Sedgwick, F.G.S., and Ro- 

 derick Impey Murchison, Esq., F.G.S. 



CAMBRIDGE PHILOSOPHICAL SOCIETY. 



At a meeting of this Society, held on Monday evening, March 

 8th, Dr. Hodgson, the President, in the Chair. 



A communication w'as made by Mr. Tozer, of Caius College, on 

 some mathematical formulae for determining the permanent effects 

 of Emigration and Immigration on numbers. The solution involves 

 a consideration of all those causes hy which the duration of life, or 

 the rate of its production, may be affected, which may be called into 

 operation by the transfer. 



Where all the elements are constant the numbers of a peopl'e, 

 after a given time, may be determined by the solution of an equation 

 of finite differences in which the coeificients are constant ; the pro- 

 bable change in the value of those coefficients may, when the data 

 are sufficient, be calculated by a method suggested by Laplace. 



Where the data are insufficient for the complete solution of the 



