a new Fossil Animal . 
565 
upper portion, and to lap over the exterior ; the rest of the outer 
face is formed, in the posterior part, by a second bone, the coronoid \ 
the bottom by a third, the angular ; and the inner face by a fourth, 
the opercular ; in addition to these four bones, the articular, which 
is placed at the posterior extremity for the purpose which its name 
denotes, and a small crescent-shaped bone, which sometimes forms 
the coronoid process, complete the stated number. 
Of these bones, we have succeeded in demonstrating the five first 
on the Ichthyosaurus, occupying situations closely corresponding to 
those which they possess in the crocodile. The crescent-shaped 
bone is the only one of the series which we have not yet detected ; 
and this probably arises from our never having seen the part of the 
jaw in which it occurs fairly exposed. 
Almost the only differences from the lower jaw of the crocodile 
are, 1st, the apparent absence of the oval hole* just behind the 
termination of the dental bone, which characterises the latter, but 
which is however not to be found in others of the lacerta family ; 
and, 2dly, which is more important, that the bones are not con- 
nected by common but squamous sutures, forming plates folding 
over one another, as in fishes. % This structure, which combines 
* In one beautifully perfect specimen of Ichthyosaurus, belonging to Mr. de la Beche, 
a sulcus communicating with a small foramen maybe seen in a similar situation. We 
have not however traced it as yet in others ; but it is so situated as to be readily con- 
cealed, by being filled up with the stony incrustation. 
+ This overlapping mode of application, or as it is technically termed, squamous suture, 
occurs also in most of the bones of the head ; since by the accidental removal of portions 
of the external bones, more or less of the internal, against which they are in consequence 
of this structure applied, is often exhibited in different specimens, a correct idea of their 
ontline can only be obtained by the careful collation of several such specimens in their 
ordinary state of preservation ; and a single one can only be relied upon where it appears 
to be undoubtedly perfect and unmutilated, a circumstance which it requires Some 
practice to ascertain. 
The sutures of some of the bones (those particularly of the orbits) are singularly 
