42 



BKITISH FOSSILS. 



from the upper margin of any part of the dorsal, to the lower margin 

 of any part of the anal fin. The greatest transverse diameter of 

 the head is equal to the distance from the snout to the posterior 

 margin of the parietal bones. 



The specimen figured in Plate II. furnishes a very complete view 

 of the structure of the cranium of Glyjptolmmus, the arrangement of 

 whose constituent elements is still further elucidated by the dia- 

 grammatic woodcuts fig. 2 (p. 2 of the " Preliminary Essay ") made 

 from enlarged and restored views of the skull and its appendages. 



The cranial bones are thin and scale-like, and their surface 

 exhibits numerous long and sinuous ridges, separated by narrow 

 and comparatively deep grooves, which sometimes obscurely radiate 

 from the centre of the bone. 



The premaxillary bones, slender and slightly curved, uniting in a 

 broad, but short, ascending internasal process, form the anterior 

 boundary of the snout and limit the nostrils below, joining the 

 equally slender maxillaries which constitute the rest of the upper 

 boundary of the gape behind. The upper and inner edges of the 

 ascending processes of the premaxillaries abut against the anterior 

 margins of a fiat hexagonal bone, whose posterior margins imite 

 with the frontals, while its lateral edges are connected with the inner 

 edges of the nasal bones. This bone is therefore obviously the 

 ethmoid. 



The frontals, which succeed the ethmoid in the middle line, are 

 short, but comparatively narrow bones, separated by a very distinct 

 suture, whicli widens in the middle of its length, so as to form a 

 small rhomboidal font?.nelle. The posterior edges of the frontals 

 are truncated, and unite with the anterior margins of the parietals, 

 which are almost twice as long as the frontals, and enter more 

 largely than any other bones into the formation of the roof of the 

 skull. The left parietal rather overlaps the right posteriorly, and 

 each parietal suddenly widens in its posterior moiety, so that its 

 outer edge presents a deep notch or step into which the post-frontal 

 fits. The posterior edges of the parietals are as abruptly truncated 

 as the anterior. They unite in the middle line with the apex of the 

 large rhomboidal scale, or bone, which occupies t])e place of the supra- 

 occipital. 



The supero-lateral regions of the skull are formed in front ■ by the 

 large nasals ; behind these by the prefrontals, which unite with the 

 maxilla, the nasals and the frontals below, in front and above, and 

 ap})arently, with the post-frontal behind. Their posterior excavated 

 margins form the anterior boundary of the orbit. 



