|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
|* In one beautifully perfect specimen of
Ichthyosaurus, belonging to Mr. de la Beche, sulcus communicating with
a small foramen may be seen in a similar situation. We have not
however traced it as yet in others; but it is so situated as to be
readily concealed, 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, mote or less of the internal, against which they arc in consequence of this structure applied, is often exhibited in different specimens, a correct idea of their outline 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 adapted for strength} being partly squamous and partly dovetailed, so that one bone is partly inserted into the other by a sort of angular process, on either side of which they overlap each other reciprocally. On these points we propose, in a future communication, to speak more in detail.