498 HAKKY GOVIER SEELEY ON THE 



parts of the skeleton of Ornithoclwirus from the Cambridge Green- 

 sand, and it closely resembles the specimen figured on pi. 11. figs. 1 

 & 2, the Ornitho saurian nature of which I see no reason to doubt ; 

 while it seemed improbable that so many specimens (as were known 

 to me) of birds' skulls should have been found, and so few of Ptero- 

 dactyles — the bones of birds being so rare, and those of Ornithosaurs, 

 especially jaws, being relatively abundant. Confessedly it is ex- 

 tremely difficult, if not impossible, to distinguish the hinder part of 

 the cranium in some Natatorial birds from the corresponding region 

 in some Ornithosaurs merely by the form of the head, the arrange- 

 ment of its regions, or the proportions of its several osseous elements. 

 When first describing the specimen referred to, I pointed out* the 

 difficulties in the way of distinguishing the fossil from the cranium 

 of a bird. The withdrawal of the specimen from the Ornithosaurian 

 list would leave the question of the Avian nature of Ornithosaurs 

 unaffected, as in most respects some of the admitted crania of Ptero- 

 dactyles are almost as bird-like as this specimen. 



I propose now to regard the skull as that of a true bird : — first, 

 because, on careful reconsideration, I find no characters by which it 

 can claim more than a generic distinction from living birds ; secondly, 

 it is more like a bird's skull than is the skull of any British Ptero- 

 dactylc ; thirdly, the cranial bones are comparatively thin and dense, 

 instead of having the cellular structure characteristic of Ornithosaurs 

 from the Cambridge Greensand, and in this character the skull agrees 

 with the other portions of the bird's skeleton. It is about the size 

 of the Red-throated Diver's cranium, and, in common with all the 

 other bird-bones found, presents a marked resemblance to that Avian 

 type. 



I have already described elsewhere f the essential features of this 

 specimen, and now offer a few measurements of its several regious, 

 which were then omitted. 



Transverse measurement over the lateral processes which form 

 the articulation for the quadrate bones, 1 ^ inch. 



Transverse measurement of the constricted squamosal region of 

 the skull just in advance of the preceding measurement, | inch. 



Transverse measurement of anterior termination of the squamosal 

 region -}| inch. 



The parietal region rapidly rises as it extends forward from the 

 occiput. 



The parietal bones, which met in the median line in a slight 

 ridge, are oblong, -^ inch long, though, owing to the anterior mar- 

 gin being concave, each becomes much wider (y 7 ^ inch) where the 

 parietal joins the squamosal bone in a nearly straight suture, T 9 ^- inch 

 below the median cranial crest. There is a bend in the upper third 

 of the parietal bone ; the superior portion looks upward and out- 

 ward ; the lower portion is more inflated than in the Diver, but is 

 similarly directed outward a little to meet the expanded frontal. 

 The parietal bone ( T x -g- inch thick) had a smooth union with the 



* Ornithosauria, pp. 80-83. 



t Ornithosauria, p. 80, pi. 11. figs. 3-6. 



