34 



BEITISH FOSSILS. 



parietal (A) which meet at an obtuse angle, the occipito -parietal 

 moiety being nearly parallel with the base of the skull, while the 

 frontal slopes obliquely forwards and downwards to the snout; the 

 occipito-parietal portion is slightly convex from before backwards, 

 and more so from side to side ; while the frontal portion, though 

 convex from side to side, is slightly concave from before backwards. 



Viewed from above, the occipito-parietal shield (A, fig. 2, 

 PL VIIL) has a trapezoidal form, being more than twice as wide 

 behind as in front, in consequence of the production of its postero- 

 lateral angles. A median sutural line distinguishes it into two 

 halves ; and, in the specimen represented in fig. 1., PI. YIII., what 

 appears to be a true suture runs obliquely from the median suture 

 outwards and forwards to the outer margin of the parieto-occipital 

 shield, cutting off a large triangular plate of bone, which appears 

 to represent the so-called squamosaV* and the suprascapular 

 {S. Sc.) of ordinary fishes from the proper parietals (P«.) 



The sculpture of this part of the roof of the skull presents the 

 form of reticulated ridges and grooves, directed more or less trans- 

 versely, with interspersed dots and splashes of enamel. 



The frontal shield (B, fig. 2, PI. VIII.), similarly viewed from 

 above, is rounded in front, somewhat constricted in the interorbital 

 region, and truncated behind, where it joins the parietals. 



Like the posterior shield, it is divided by a median longitudinal 

 suture, and in the specimen figured in PI. VIIL it presents indica- 

 tions of the existence of a transverse dentated suture at its most 

 constricted part. 



In the same specimen, which is'^perhaps young, the proper frontal 

 bones appear to be very narrow, the outer third of each being 

 formed by a fringe of apparently distinct marginal ossicles (a;) ; but 

 these seem, eventually, to become completely united with one 

 another, and with the frontals. The surface of the frontal shield 

 exhibits a pitted and reticulated sculpture, like that of the occipito- 

 parietal, which is most distinct on the marginal ossicles. The dots 

 of enamel are scanty, and scattered at wide intervals. 



The basis cranii («, 5, c) is formed by a layer of bone, which is 

 continuous, and presents no trace of sutures, between a and h. 

 Behind b it is defective for some distance, but reappears in front 

 of c. It doubtless represents, in the greater part if not the whole 

 of its extent, the parasplienoid of ordinary fishes. 



Between the orbits this parasphenoid has the form of a stout bar 

 of bone, grooved above, convex and smooth below ; but it becomes 

 flattened out from above downwards, both anteriorly and posteriorly. 

 Anteriorly, it ends as a spatulate plate (Plate VIIL, fig. 3«), 

 which has prominent lateral margins, bounding a transversely con- 

 cave under surface, over which are scattered multitudes of minute 

 granular teeth, those on the margins of the plate being some- 

 what larger than the rest. The dentigerous plate may represent 

 a vomer, but in no specimen that I have seen can it be distin- 

 guished from the parasphenoid. 



