CRETACEOUS FORMATIONS. 13 



ossified, plate, representing the sternum of struthious birds. The bony sternum, 

 or " episternum," is long, narrow, and depressed; it is considerably produced 

 in advance of the coracoids, but this produced part is flattened horizontally. 

 If it be compared with the pre-coracoid part of the sternum in the Pterodactyle 

 or penguin, it is not more like the one than the other. In the main, the 

 Pterosaurian breast- bone, like the scapular-arch, is formed on the ornithic type, 

 but the post-coracoid, lateral emarginations are distinctive Pterosaurian characters. 



The Humerus of Pterodactylus (Tab. III). 



The fragile texture of the bones of the Pterodactyle, and the consequently 

 crushed or broken state in which those of the wings more especially have hitherto 

 been usually found, have precluded any precise description or figures of the 

 articular surfaces, or of the configuration of the extremities of these bones. 

 And yet such particulars are absolutely requisite for defining the resemblance 

 of the Pterosaurian humerus to that of the bird and reptile, and for acquiring 

 this element in the determination of the degree of affinity or relation of the 

 Pterosauria to those classes respectively. 



The remains of the very large species of Pterodactyle from the Cretaceous 

 formations of Kent and Cambridgeshire have furnished materials for advancing 

 this desirable knowledge in regard to the structure of the vertebrae,* and I have 

 now similar means of contributing more precise information respecting the 

 structure of the proximal end of the humerus than has hitherto been possessed. 

 For the subjects of this study and comparison I am chiefly indebted to Professor 

 Sedgwick. But, in proceeding to impart the results, I must premise some notice 

 of the character of the humerus in birds, in which I shall avail myself of the 

 terms indicative of aspect and position proposed by Dr. Barclay, in his 

 ' Anatomical Nomenclature.' 



Proximal signifies the upper, distal the lower, ends of the bone, as it hangs in 

 man ; anconal is the posterior, palmar the anterior, surface, as when the palm 

 of the hand is directed forward ; radial is the outer, tdnar is the inner, side, 

 according to the same position of the human arm and hand. Proximad, palmad, 

 &c, are adverbial inflections, meaning towards the proximal (upper) end, and 

 towards the palmar (anterior) side. 



In the bird, then, the humerus has a smooth shaft, sub-elliptic in transverse 

 section, with expanded ends, the proximal one being the broadest. Lengthwise 

 the bone is gently sigmoid, the proximal half being convex palmad, the distal half 



* Phil. Trans., torn. cit. 



