38 



BRITISH FOSSILS. 



In the mandibles, which are represented in PI. VII., the greater 

 part of the angular-articular and dentary elements have been broken 

 away leaving only the plate which answers to the splenial of reptiles 

 (fig. 3., SpL) And it is this plate which gives rise to the height and 

 straightness of the upper edge of the middle third of the ramus. 



The angulo-articular element of the mandible, which forms the 

 outer part, if not the whole, of the ramus at the articulation, 

 rapidly diminishes in height, and leaves the outer surface of the 

 splenial bare as it passes forwards. It then meets the dentary, 

 fitting into a V-shaped space afforded by the latter. 



The dentary, passing forwards, suddenly rises into a kind of 

 shoulder, y (fig. 4, PL VII.), which applies itself to the anterior part 

 of the outer surface of the splenial. 



In uninjured specimens the whole outer surface of the angulo- 

 articular is covered with tubercles of enamel. Similar, but more 

 scattered, tubercles ornament the dentary ; but the outer surface 

 of the splenial, exposed between the process r/ and the front and 

 upper margin of the angular-articular, is perfectly smooth. 



There is a distinct suborbital half ring (PI. VII., fig. 1), formed 

 to all appearance of a single bone, or of several bones which have 

 coalesced, and presenting a granular external sculpture. The 

 posterior part of the suborbital ring is much broader than the 

 anterior, and abuts upon the operculum, behind and below which it 

 comes into contact with the triangular postmaxillary bone ; in 

 front and above it, exhibits a deep notch. 



The ornamentation of the suborbital bones consists of pits and 

 reticulated ridges, with scattered tubercles of enamel. 



The operculum (PI. VII., fig. 1, op.), is a large four-sided bone. 

 Its upper margin is shortest ; its front margin, which is as long 

 as the distance from the roof of the skull to the lower edge of the 

 triangular post-maxillary " bone, longest. The posterior margin 

 is much shorter, so * that the lower edge of the operculum runs 

 very obliquely, from above and behind downwards and forwards. 



Traces of a suhoperculum, much smaller than the operculum, are 

 discoverable in some specimens. 



When the outer surface of the operculum is complete, it is 

 covered with close-set conical enamel tubercles, like those of the 

 roof bones of the cranium. 



Two large jugular plates occupy the whole interval * between 

 the rami of the mandible. The inner, opposed, margins of these 

 plates are straight, the outer, evenly convex. 



The hyoidean arch is strong and well ossified, and is connected 

 with the hyomandibular by a very strong stylo-hyaV^ {St h., 

 figs. 3 and 6, PL VII.). 



Each branchial arch is, apparently, a single arcuated bone, deeply 

 grooved posteriorly. I can count only four on each side in the 



* In No. 25,872, of the British Museum Collection, the surface of the jugular 

 plates is perfectly preserved, and is ornamented with tubercles of enamel, *^hich 

 are set, evenly, and pretty close together, over its whole surface. 



