608 



MB. J. W. DAVIS OX CHOXDKOSTEUS ACIPEXSEKOIDES. 



anterior margin of the operculum there was, probably, attached an 

 inter-operculum ; but though its place appears to be indicated, there 

 is no trace of the bone remaining. 



The frontals, postfrontals, parietals, mastoid, and some of the 

 occipital bones are present. There is indication of an extension of 

 the front als on a portion of the matrix, which extends about an inch 

 forward from that part of the dermal bones actually preserved. 

 All the bones are united by sutures. The frontals are 2*5 inches 

 in length and 1*0 inch in breadth ; they are broadest posteriorly, 

 where they join the parietals and mastoid. Attached to the inferior 

 margin of the frontal is the postfrontal (sphenotic of Parker), 

 shorter than the frontal, and extending downwards so as to embrace 

 the upper margin of the orbit ; its posterior margin articulates 

 with the mastoid. The mastoid is a large plate, with a length of 

 3*0 inches ; it is 1*3 inch in width near the posterior extremity, 

 diminishing forward to 1*0 inch. The centre of the plate is 

 considerably raised and forms a ridge ; from this the surface 

 declines to the extremities, which are much thinner and are jagged 

 and uneven where attached to the neighbouring plates. The 

 external surface of the mastoid is coarsely striated or ridged. The 

 ridges radiate, for the most part, from the centre towards the 

 margin ; the surface of the ridges is covered by strips of ganoine, 

 apparently thin, and not extending to the base of the intermediate 

 striations. The parietal plates are equal in size to the frontals ; 

 they are rhomboidal in outline, roughly striated on the surface, 

 the striae radiating from a slight elevation of the median portion. 

 Connected with the posterior margin of the parietal plate there 

 is a long and narrow bone. It extends backwards 3'0 inches, 

 and is 0*5 inch in width. It presents the appearance of being 

 a lateral bone, having a complementary one on the opposite 

 side of the head, to which it is anchylosed or joined by suture. 

 In the recent Sturgeon a single median bone terminates the 

 cranial coveiing, and is succeeded by the closely-imbricating series 

 of scutes, which extend along the dorsal surface of the body. In 

 the fossil there is no evidence of such a series of dermal scutes, 

 and this posterior portion of the dermal covering of the cranium is 

 proportionately much elongated ; the lateral surfaces of the com- 

 bined plates are produced along the median line, and form a raised 

 and acutely angular ridge. The posterior surface of the plate is 

 covered with coarse pustulations of enamel ; its anterior portion 

 envelopes, and apparently affords attachment to, the upper extremity 

 of the suprascapular. The bones occupy the position, and are pro- 

 bably the equivalents, of the supraoccipital (8). Another bone, 

 thin and widely expanded, whose posterior margin is overlain by 

 the suprascapular, whilst its anterior one lies hidden by the dermal 

 bones of the postorbital region of the skull, from its position 

 must represent either the exoccipital or the petrosal portion of 

 the base of the cranium, and may have been connected with the 

 basisphenoid by cartilage when living, though now separated by a 

 distance of 0*25 inch. 



