390 Miscellaneous. 



inwards and forwards as a slender process to meet the oblique parietal 

 ridge. The lower ray is a broader plate, smooth and slightly convex 

 outwards, forming the extreme sides of the occipital region ; articu- 

 lating outwardly with the tympanic, and below with the parocci- 

 pital and pterygoid. The outer ray is a narrower plate, smooth and 

 slightly convex externally, forming the upper and hinder longitudinal 

 half of the deep zygoma, uniting anteriorly with the post-frontal 

 and externally and inferiorly with the bony plate, described as the 

 " squamous element " of the temporal, in my ' Report on Fossil 

 Reptiles,' 1839, but which I now regard as an accessory sclero- 

 dermal bone, analogous to, but not homologous with, the squamous 

 plate in man and mammals. 



The prespheuoid is a long slender trihedral bone, broadest where 

 it joins the basis])henoid, with the two lower sides converging to- 

 wards a median obtuse angle. It divides the long and narrow 

 pear-shaped spheno-pterygoid vacuities. 



Of the orbitosphenoids I have no accurate cognizance : they may 

 not have been ossified. The frontals are remarkable for their small 

 size, especially in length as compared with breadth, in which one 

 cannot but be reminded of their cetaceous type. The median part of 

 each frontal is convex, the lateral part is concave, notched behind 

 for the parietal, and more deeply in front for the nasal, the process 

 entering which being separated by a very narrow strip of the frontal 

 from the opposite process of the parietal. Laterally the frontals 

 unite by a straight margin directed obliquely outwards and forwards 

 with the post-froutals. 



The post-frontal much exceeds the frontal in size ; it extends from 

 the upper surface of the cranium horizontally, on each side the frontals, 

 to an equal extent with those bones, then bends obliquely down to 

 form the upper and anterior half of the zygoma. The horizontal 

 plate is semi-elliptic, convex in front, a little concave behind, where 

 it defines anteriorly the temporal fossa. The inner angle overlaps 

 and articulates with the antero-external angle of the parietal ; the 

 inner border joins the whole outer border of the frontal, touches 

 the outer point of the nasal ; the convex fore-part joins the pre- 

 frontal and completes the rim of the orbit above. The zygomatic 

 branch unites with the mastoid behind, and with the super-squa- 

 mosal and post-orbital below. 



The similarity of character in the post-frontal and mastoid is 

 instructive in regard to their general homology. 



The prefrontal, I, as yet, know only by its external or facial part. 

 This is a narrow, moderately long, curved bone, extending from the 

 post-frontal to near the nasal aperture, receiving there the upper 

 angle of the lacrymal in a notch, the upper boundary of which is 

 wedged between the lacrymal and nasal : with the latter bone it is 

 in connexion along its whole inner border ; it does not join the 

 frontal, and its position and relations in the Ichthyosaurus, as in 

 some fishes, instructively illustrate the true nature of the prefrontal 

 as an element of a cranial segment distinct from that to which the 

 frontal belongs ; and not as a mere dismemberment of the frontal 



