CRETACEOUS FORMATIONS. 13 



The most perfectly preserved of the lateral impressions (fig. 6) is of an oval form, 

 1 inch 3 lines in long diameter ; it is well defined from the narrower upper surface 

 (fig. 7) to which it stands at nearly a right angle ; the curved border defining it is not 

 produced. The whole of the substance of the bone between the lateral plates is 

 occupied by a moderately open and apparently pneumatic cancellous texture (fig. 8) ; 

 the outer wall of bone is compact, but extremely thin ; the general structure is 

 decidedly that of a volant Vertebrate, and most resembles that of a Pterodactyle. 



The parts of the skeleton of the Pterodactyle which would afford a symmetrical 

 median piece of bone, comparable with the present fragment, are — the sternum, the 

 fore part of the upper and lower jaw, the sphenoid at the base of the skull, and the 

 parietal and frontal bones at the upper part of the skull. The absence of any trace of 

 cranial cavity at the lower fractured surface, more than an inch below the outer 

 surface, opposes the choice of the parietal with lateral impressions of temporal 

 fossae : there remains, therefore, the frontal with the interpretation of the lateral 

 depressions as parts of the orbits ; but the depth of the smooth impressed plates, and 

 their divergence as they descend, oppose this interpretation. I have no evidence of 

 sternal ends of coracoids which would require articular depressions of such size and 

 shape as the lateral ones on the fragment in question, on the hypothesis that it may 

 be from the fore part of the sternum. Upon the whole, therefore, I have to acknow- 

 ledge a degree of uncertainty as to the exact nature of the present fragment of the 

 skeleton, most probably, of some large Pterodactyle. 



Scapular Arch. 



The mechanism of the framework of the wings in the Pterodactyle is much more 

 bird-like than bat -like. The scapular arch is remarkably similar to tbat of the bird 

 of flight. It consists of a scapula and coracoid, usually anchylosed where they combine 

 to form the shoulder-joint. 



The cavity for the head of the humerus, in Pterodactylus macronyx * (Tab. Ill, 

 fig. 6), is oval, with the great end formed by the scapula; it is concave verti- 

 cally, or in the direction of its long diameter, convex transversely, but least so 

 near the scapula. If these proportions hold good in other species, they would 

 serve to determine the scapular or coracoid portion of a glenoid cavity, when, as in 

 the case of the fossils here described, the rest of the scapular arch had been 

 broken away. 



The upper (scapular) border of the glenoid cavity is prominent and well defined ; 

 the bone is moderately constricted beyond it, from without inward, whence the 



* Buckland, 'Geological Transactions,' 2d series, vol. iii, pi. xxvii, x, 9. 



