462 GLYPTOLZMUS KINNAIRDI 
great as the distance from the end of the snout to the posterior margin 
of the opercular apparatus ; which distance exceeds by as much as a 
fourth, or a fifth, the transverse diameter of any part of the body. It 
somewhat exceeds, again, the perpendicular distance 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. [Plate 34] furnishes a very com- 
plete view of the structure of the cranium of G/yptolemus, the 
arrangement of whose constituent elements is still further elucidated 
by the diagrammatic 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 compar- 
atively 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 bound- 
ary 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 flat hexagonal 
bone, whose posterior margins unite 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, which widens in the middle of its length, so as to form a small 
rhomboidal fontanelle. The posterior edges of the frontals are trun- 
cated, 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 the place of the supraoccipital. 
The supero-lateral regions of the skull are formed in front by the 
large nasals ; behind these by the prefrontals, which unite with the 
