580 Mr. De la Beche and Mr. Conybeare on 
Comparing the proportions of this head with that of the entire 
skeleton figured in Phil. Trans, for 1819, it must have belonged to 
an animal exceeding twenty-four feet in length. 
The geological seat of these interesting remains is principally in 
the lias, and in the clay underlying the Portland oolite at Kimmeridge, 
and on Shotover hill, near Oxford ; but they have also been found 
in some of the intermediate formations, e. g. in the calcareous grit 
underlying the Oxford oolite at Marcham near Oxford; and again 
in one instance in a formation still more recent than the Kimmeridge 
clay, namely in a bed of marie associated with the green sand series 
at Bensington in Oxfordshire, probably contemporaneous with that 
of Folkstone in Kent. On the whole, therefore, it appears that the 
occurrence of these animals is at least coextensive with the whole 
series of formations intervening between the new red sandstone 
and chalk : they seem however to be most abundant in the lias. 
We have seen specimens from numerous localities situated on 
that formation in Dorsetshire, Somersetshire, Gloucestershire, and 
Leicestershire. 
It has been our object, in drawing up the preceding details on 
the genus Ichthyosaurus, to establish a firm foundation of anatomical 
knowledge, which might be applied to the illustration of the newly 
discovered genus plesiosaurus, to which it is obviously in many 
respects allied. For since, of the latter, comparatively few speci- 
mens have been yet accumulated, and those deficient in many 
important members, we are not as yet in a state to neglect the aids 
to be derived from such analogies. 
To communicate a notice of the discovery of this new genus 
was the principal object of the present paper; yet, from the cir- 
cumstances above mentioned, our details concerning it will necessarily 
occupy a far less space than those which strictly speaking were 
