28 OSTEOLOGY OF PTERANODON. 



spaces. Similar changes would probably take place in the expiratory muscles attached 

 to the anterior vertebral ribs. On the other hand, those muscles of the thorax and 

 abdomen controlling the movement of the sternum and sternal ribs would be developed 

 in proportion to the respiratory needs of the animal. 



WING BONES. 



Of all the skeletal parts of Pteranodon, the wing bones perhaps offer the least in- 

 ducement to further investigation. Obtained more abundantly at the outset by collec- 

 tors, and usually in better preservation than the axial skeleton, they naturally form the 

 subject of much of the earlier contributions to the osteology of this genus, with the 

 result that their study has been carried nearly to completion. It is proposed in this 

 memoir to figure the entire skeleton as far as it is possible to do so from specimens in 

 the Marsh Collection, and some brief comment upon the wing bones may properly 

 accompany their illustration without attempting to paraphrase descriptions, the excellent 

 definitions recorded by Marsh, Williston, and Plieninger being easily accessible. 



In the introductory pages of the present memoir, Professor Marsh's separation of the 

 two species P. occidentalis and P. ingens is shown to have been based partly upon the 

 relative size of the types and partly upon differences in the form of their humeri and 

 of the proximal ends of their ulna;. It is also stated that further work upon the collec- 

 tion has led the writer to the conclusion that these differences are of no taxonomic 

 value, as they appear to be merely the variant results of unequal pressure in the matrix. 

 It is impossible to record anything more satisfactory in regard to the proximal articula- 

 tions of the ulnae in question than the statement that these bones having been so 

 greatly distorted, the wonder is, not that the original investigator gave undue impor- 

 tance to their differences, but rather that he so promptly recognized in them the same 

 skeletal element. 



In regard to the supposed specific difference of the two humeri, it is now possible 

 to write more explicitly. When the humerus of P. occidentalis (type), No. 1164, was 

 described by Professor Marsh, the " anconal " side alone was entirely exposed to view 

 (Plate XIX, figure 9). He was therefore deceived by the appearance of the radial crest, 

 which then seemed much smaller than the corresponding process of the humerus of 

 P. ingens. Upon removing the matrix from the opposite side of P. occidentalis (Plate XIX, 

 figure 10), it became evident that the radial crest had been crushed and shortened by 

 pressure, and that no specific difference could be pointed out in these processes of the 

 two types. The humerus of P. ingens (type), No. 1175, is somewhat better preserved 

 than that of P. occidentalis, which it resembles closely except in size, and although the 

 radial crest is by no means perfect, the generic characters of the bone are well dis- 

 played. Two views of this humerus are given, Plate XIX, figure 1, showing the 

 '• anconal " or dorsal surface, and figure 2, the palmar or inferior surface. 



The vagaries of form assumed by the humerus under pressure in the matrix are 

 surprising, the first result of this perplexing situation being that almost every humerus 

 in the collection seems to represent a distinct species. From an examination of four- 

 teen practically complete humeri of Pteranodon, variously distorted, it appears that 

 pressure in the vertical direction (the vertebral axis of the pterodactyl being supposed 

 to lie in the horizontal plane, with the wings outstretched laterally) usually crushes 

 and shortens the radial crest, while pressure in the horizontal plane not only leaves the 



