SHOULDER GIRDLE. 27 



the sterno-costalia of Campylognathus zitteli, figured by Dr. Plieninger in : " Die Ptero- 

 sauria der Juraformation Schwabens," p. 222. 



The quotation from Professor Williston's " Restoration of Ornithostoma," in regard to 

 the sternum, conveys the idea that one or more "coossified" vertebral ribs issuing from 

 the anterior dorsal vertebrae were connected directly with the sternum ; and the same 

 eminent paleontologist has written concerning the more or less closely allied genus 

 Nyctosaurus (Osteology of Nyctosaurus, p. 136) : " There are apparently four pairs of stout 

 ribs arising from the first four dorsal vertebrae, the first three of which, at least, are 

 anchylosed to the centrum in the adult animal. They were doubtless all attached to 

 the four tubercles on each side of the sternum." And again in the same work, p. 139 : 

 "It has been suspected that there are sternal ribs intervening between the vertebral ribs 

 and the sternum, but there is no evidence of such in the present specimen." 



The notarium of Pteranodon sp., No. 2692, already figured, proves beyond all doubt 

 that the vertebral sections of the first three dorsal ribs are anchylosed to their vertebrae. 

 If the lower ends of these vertebral ribs articulated directly with the lateral sternal pro- 

 jections, the sternum would be immovably fixed in its position ; and, no account being 

 taken of diaphragmatic breathing, which is at the most but rudimentary and negligible in 

 Aves and Reptilia. this pterodactyl could have filled its lungs only by gulping air into 

 them as do the Chelonians with fixed plastron. On general grounds, such limited res- 

 piratory action would seem inadequate in a flying reptile, the pneumaticity of whose 

 skeleton almost or quite equaled that of the most lightly-framed existing bird. 



More direct evidence on the question of the fixity or mobility of the sternum is offered 

 by the peculiar saddle-shaped facets at the base of the manubrium sterni, for the artic- 

 ulation of the lower ends of the coracoids, a joint which Professor Williston himself 

 stated " must have had considerable mobility " (see p. 26). There is no doubt whatever 

 that Professor Williston is correct in his statement that the sternum moved upon the 

 coracoids, although such a conclusion appears at variance with his ideas on the struc- 

 ture and connections of the dorsal ribs. The mechanical principle involved is simple, 

 and from the diagram of the thorax (Plate XXXI), it will be readily seen that not only 

 must well-developed sternal ribs have intervened between the anterior vertebral ribs and 

 the sternum, but also that there must have been a slight rotation of the scapulae upon 

 the supra-neural facets of the notarium ; otherwise there could have been no movement 

 of the sternum. Even then the mechanical action of respiration would not be exactly 

 equivalent to that prevailing in many reptiles and in birds, where both the vertebral ribs 

 and the corresponding sternal ribs are free to move when drawn forward by the inspira- 

 tory muscles, the main action of these muscles being to straighten the angles between 

 the vertebral and sternal sections of the ribs, and thus by the well-known principle of 

 the "toggle-joint" to force the sternum away from the vertebral column. 



In Pteranodon, on the other hand, the action of the thoracic mechanism was greatly 

 lessened by the immobility of the anterior vertebral ribs, as well as by the complete 

 anchylosis of the coraco-scapular union, an articulation which permits of considerable 

 movement in almost all carinate birds ; and the movement of the sternum was accom- 

 plished by swinging it bodily downward and backward about the distal ends of the 

 vertebral ribs, to which points it was united by sternal ribs. Clearly, in this animal, 

 the anterior vertebral ribs being fixed, the levatores costarum and the external inter- 

 costals would cease to function as contractile muscles, and in the adult pterodactyl they 

 would probably degenerate into aponeurotic tissue, serving only to close the intercostal 



