AN ICHTHYOSAITKUS FROM THE LIAS OF WHITBY. 637 



intervening inner part of the postfrontal bone (pt), which juts in 

 almost at a right angle. The anterior terminal processes referred 

 to divide the postfrontal hones entirely, or almost entirely, from the 

 frontals, and themselves unite with the nasal bone (n). The tem- 

 poral fossae (TF) have not been sufficiently excavated to show the 

 depth of the parietals or their relations to the lower bones of the 

 brain-case. In front of the parietal are the frontal bones (/), 

 rather narrower than usual, and having a length of 4|- inches. 

 They are less than ^ of an inch wide posteriorly, where they 

 are truncated by the parietal foramen. They are widest at the an- 

 terior termination of the parietal bones (_p) ; and posterior to this 

 their converging lateral outlines, though somewhat irregular, are 

 gently concave. The anterior lateral outlines are less than twice as 

 long as the posterior part, are straight, and converge to a point, so as 

 to give the bones somewhat the outline of a spear-head. They are 

 each a little convex from side to side, and are divided by a median 

 suture, which is most impressed in front. The posterior parts of the 

 bones articulate exclusively with the parietals, the anterior part 

 exclusively with the nasal bones, which they gently separate pos- 

 teriorly. 



The nasal bones (n) are, as usual, the great roof-bones of the fore 

 part of the head. Their anterior termination is not distinctly seen ; 

 for the upper surface has suffered an injury by which their extre- 

 mities have been removed. Measuring from the angle between the 

 parietal and postfrontal bones to the apparent anterior extremity, 

 beyond the visible termination of the nasals, the distance is about 

 17 inches. As already remarked, the nasals are angularly bent, so 

 that the upper portions form the roof of the skull, and outer oblique 

 parts form the upper border of the side of the skull in front of the 

 eye and above the nares. The posterior half of the upper surface 

 is concave from side to side, having a width in the hinder region 

 of about 4 inches. The outline of the nasal bones is necessarily a 

 long spear-shaped triangle, tapering in front and wide behind. 

 They form in the middle of their own external borders part of the 

 upper wall of the anterior nares, but are not indented by them ; 

 they expand a little by descending on the lachrymal bones (Z) to 

 form the upper margin of the posterior border of the narial cavities. 

 They then have a sinuous union, at first concave and then convex, 

 with the superior borders of the large lachrymal bones, and behind 

 these pass interior to the small prefrontal bones on to the post- 

 frontal. Each bone divides posteriorly into two forks : the inner 

 wedge-shaped pair have their sides converging to blunt points sepa- 

 rated by an interspace of 4:^^ inches ; they join the frontal, parietal, 

 and postfrontal bones. These inner processes are divided from the 

 outer pair, which are directed outward and backward, though not 

 quite so far as the others, by a triangular forward wedge of the 

 postfrontal bone extending between them ; and this outer process of 

 the nasal, which is only preserved on the left side, has its margins 

 nearly parallel, and terminates in three claw-like digitations. It 

 prolongs backward the line of the rounded angular ridge running 

 along the nasal bone. These ridges are marked with a few longitu- 



