DORSAL VERTEBRA. 19 



In Pteranodon sp.. No. 2451, which is a little smaller than No. 2692, there are preserved 

 the last two notarial vertebrae so completely fused that they are as one bone ; next, 

 three separate dorsal centra, and following these the four anterior centra of the sacral 

 series inseparably fused together (Plate XII, figure 1). While this sacrum is manifestly 

 incomplete, enough remains to show that ten vertebrae enter into its composition, three 

 of them anterior to that vertebra whose transverse processes share in the formation of 

 the anterior pelvic brim. Without anticipating further the discussion of the sacrum, it 

 seems best to designate in this way one of the sacral vertebrae which will serve as a 

 fixed point in calculating the number of dorsal vertebrae. 



Again, in the type of P. ingens, No. 1175, there are preserved three dorsal vertebrae. 

 In these, the articular surfaces of the centra and of the zygapophyses indicate a little 

 movement, which of course argues in favor of their supposed location between notarium 

 and sacrum. The sacrum of this type, to be described later, is composed of a thoroughly 

 fused column of ten vertebrae, the fourth member of the series bearing the transverse 

 processes that partly define the anterior pelvic brim. No. 2451 and No. 1175 thus appear 

 to differ from No. 2616 in having only three free dorsals. There is, however, not the 

 slightest evidence that the total number of vertebrae is variable. The possible existence 

 of a fourth free dorsal in No. 2616 is best explained by referring to the fragmentary 

 pelvis of Pteranodon sp., No. 2489 (Plate XIII, figure 3), in which the sacral vertebrae 

 number but nine, the third member of the series giving rise to the transverse processes 

 which enter into the formation of the anterior pelvic brim. This matter will appear 

 more clear after the pelvis has been described in detail. 



The series of notarial vertebrae finds its counterpart in the consolidated dorsal verte- 

 brae of many recent birds ; and as the musculature of birds and reptiles is also much 

 the same, a study of the avian dorsal region will help toward a better understanding 

 of the notarium of Pteranodon. The subject can be best introduced by quoting from 

 Newton's "Dictionary of Birds," p. 854: "In many birds the thoracic vertebrae shew a 

 tendency to more rigid junction, which is often effected in old individuals by the ossi- 

 fication of the various ligaments connecting the processes of adjoining vertebrae, or even 

 by the ossification of the attached tendons of the spinal muscles. In other cases con- 

 solidation is carried further by the co-ossification not only of the centra but also of the 

 spinous, transverse and zygapophysial processes of adjoining vertebrae, so that in ex- 

 treme cases the whole dorsal region may become one continuous mass of bone." 



If the eminent writer of the foregoing quotation had been describing the notarium of 

 Pteranodon, his words could hardly have been better chosen, so nearly does the ptero- 

 dactyl resemble the extreme case cited. This coalescence of dorsal vertebrae in the 

 avian sk'eleton is generally regarded as directly correlated with increased power of 

 flight. Such was clearly the view of Sir Richard Owen when he wrote (Comparative 

 Anatomy and Physiology of Vertebrates, Vol. II. p. 17): "The neural spine is a compressed 

 quadrate plate, its truncate summit is often thickened, sometimes produced forward and 

 backward to fix the vertebrae from their highest points ; ossified tendons of spinal mus- 

 cles, also, aid the coalesced spinous and transverse processes in fixing part of the dorsal 

 region, but only in birds of powerful flight, and not in all such. The partial anchy- 

 losis of the dorsal region is associated in Falcons with their 'hovering 1 action." The 

 exception to the rule is happily stated, for in the Frigate-bird, of extremely wide wing- 

 spread and noted for its powers of flight, the vertebral column anterior to the pelvis 

 retains its flexibility to advanced age, if not throughout life, the vertebrae remaining 



