574 
Mr. De la Beche and Mr. Conybeare on 
cannot be less than between eighty and ninety, that number being 
exhibited on the specimen figured in the Phil. Trans, for 1819, 
and as several are there displaced, it is probable that some are 
missing. 
The changes in their structure corresponding with and indicating 
their places in the vertebral column, are represented in the accom- 
panying figures from a nearly entire column in the possession of 
Colonel Birch. 
The atlas and axis* have not yet been clearly ascertained ; but 
there seem to be traces of them in the first bones of the column in 
question. At any rate, it is highly improbable that more than one 
or two of the anterior vertebrae can be missing (if any are so) in 
that specimen. The numbers attached to the figures (fig. 12, plate 41,) 
indicate their place in that series. 
The seventeen first have but one tubercle ; they are farther 
distinguished by the broader and peculiar form of the sockets into 
which the annular part articulates, which in all the remaining ones 
is straight and narrow. The third vertebra has been selected as a 
specimen of these, and the seventeenth or last of them to shew the 
transition to the next form ; these are evidently cervical and anterior 
dorsal. 
The false ribs and ribs connected with these vertebrae must have 
articulated by a double head, partly against the tubercle, and partly 
* We have seen only the inferior piece or body (if it can be so called) of the atlas; 
and the odontoid process (which as in all the reptiles forms a distinct piece) of the atlas ; 
they very nearly resemble those of the turtle ; but we refrain from figuring them till we 
shall have seen the parts entire. There seem to be some specific differences in the form 
of the tubercles in different vertebral columns of the Ichthyosaurus, perhaps they may 
also be affected by age ; they are not however such as to affect the general arrangement 
and position of them as described in the text ; all the small specimens which we have 
seen, which are generally the most sharply marked, agree w r ith those we have figured. 
