FROM THE LOWER LIAS OF CHARMOTJTH. 



471 



length taken as 100, we shall obtain its latitudinal and altitudinal 

 indices. These are given in the following Table for the cervical 

 vertebrae of a number of species. To make their comparison of 

 value, corresponding vertebrae should be selected for each species ; 

 and for this tables like that on p. 452 would have to be constructed 

 and discussed. As it is, I have had to make the best use I could of 

 the material ready to hand in published papers, and to trust to the 

 chance of different describers having given measurements of an 

 average cervical vertebra. Most of the indices in the table have 

 been derived from the thirteenth to the fifteenth vertebrae, and 

 probably are sufficiently comparable. 



Plesiosaurus 



Latitudinal 

 index. 



Altitudinal 



index. 

 



Number of vertebra. 



Conybeari 



70 



100 



XV. 



hoinalospondylus 



92 



72 



XIII & XIV. 





107 



100 





coelospondylus 



110 



106 



XV. 





112 



84 



XV. 





112 



100 



Middle. 





113 



101 | 



Average of middle and 

 basal half of neck. 





113 



94 







116 



116 





carinatus 



120 



98 





sp. from Aust 



123 



113 







125 



112 





validus 



127 



102 





eleutheraxon 



127 



100 





costatus 



134 



119 







140 



109 







140 



116 





pachvomus 



142 



110 





trigonus 



150 



120 







169 



100 







170 



155 



XV. 



It will be seen that the centrum of the fifteenth vertebra of P. 

 Conybeari has a lower latitudinal index than any other known spe- 

 cies, while its height remains about the average. 



P. liomalosponclylus, which makes the nearest approach to it, is 

 equally remarkable for its exceptionally low altitudinal index. 



The compression which the cervical vertebrae of P. Conybeari have 

 undergone may to some extent account for the narrowness of their 

 centra ; but it cannot be altogether explained in this way, since it 

 persists in a marked manner down to the twenty-eighth vertebra. 

 Moreover our measurements were taken from the articular ends of 

 the centra; and these show no obvious signs of compression. 



The Table shows a general tendency in the long-necked species 

 towards a low latitudinal index, and in short-necked species towards 

 a high one ; but the rule is subject to exceptions, and we have not 

 enough instances to reason from. The latitudinal has clearly a 

 greater range of variation than the altitudinal index. 



2i2 



