22 OSTEOLOGY OF PTERANODON. 



gallipavo 1 (Y. U. Osteol. Coll., No. 2112), shown in Plate IX, figure 4. Morphologically 

 considered, the fourth synsacral vertebra of P. ingens, No. 1175, is clearly the homologue 

 of that vertebra in the avian sacrum here figured, whose transverse processes aid in 

 the formation of the incomplete anterior pelvic brim, and also bound anteriorly the fovea 

 containing the kidneys. The three following vertebrae of P. ingens, numbers 5, 6, and 7, 

 occupy precisely the same position as those avian vertebrae which Huxley identified as 

 true sacrals, on the ground that the nerves issuing from their intervertebral foramina 

 form the sacral plexus. It has been already pointed out that vertebras 8, 9, and 10 of 

 the synsacrum of P. ingens support the posterior extensions of the ilia both by their 

 transverse processes and also by their neural spines, in this regard differing from ver- 

 tebrae 5, 6, and 7, and resembling the " posterior parapophysial vertebrae '' of the avian 

 sacrum, to adopt the phrase used by Messrs. Mivart and Clarke, in their treatise : " On 

 the Sacral Plexus and Sacral Vertebrae of Lizards and other Vertebrata" (Linn. Soc. 

 Trans., 2nd series (Zool.), vol. i, p. 513). The following quotation from the scholarly 

 work of these gentlemen is of especial interest in this connection : 



" The determination of the homological relations of the different parts of the post- 

 dorsal part of the spinal column of Birds is a matter of much difficulty. It might be 

 anticipated, however, from the close relations and probable genetic affinity between Birds 

 and Reptiles, that the true sacral vertebrae of the former class might be readily deter- 

 mined by the aid of reptilian characters. But however close may be the morphological 

 affinities, they are so strangely disguised by great physiological differences, that the 

 results of comparison are much too unsatisfactory to justify the anticipation above re- 

 ferred to. The enormous forward prolongation of the iliac bones and the great power 

 and activity of the erect and vigorous legs of birds, compared with the backwardly ex- 

 tending ilia and the sprawling and feebly pushing legs of Saurians, combine to produce 

 in Birds a redundancy of both osseous and nervous structures compared with those of 

 the existing Reptilia. Could we dissect the different forms of Pterosauria and Dinosauria. 

 these difficulties and obscurities would no doubt disappear; but the hiatus is too great 

 between the most Sauroid of existing Birds and any form of Lizard for the homological 

 relations, in this respect, to be readily and easily solved." 



Returning to Pteranodon, certainly no form of reptilian sacrum could be desired by 

 Paleontologists, which should approach more closely the common type of avian sacrum. 

 The difficulty lies not in comparing Pteranodon's sacrum with that of Birds, but in reconcil- 

 ing its peculiar specialized development with the simpler forms known in other Reptilia. 

 Since the eighth and ninth vertebrae of the synsacrum of P. ingens so closely resemble 

 the two " posterior parapophysial vertebrae " of the avian sacrum, it will be instructive 

 to compare the sacrum of P. ingens with that of an older and more primitive pterosaur. 

 Quenstedt's plate of Pt.erodactylus (Cycnorhamphus) suevicus, published in 1855 (Ueber 

 Pterodactylus suevicus), shows two anchylosed vertebra? whose transverse processes unite 

 distally to support the ilia. Immediately in front of these two sacrals is a somewhat 

 longer vertebra, described by that author in the following words : 



" Nicht minder interessant als das Kreuzbein ist der starke Wirbel Nro. 22, man konnte 

 ihn den Kreuzbeintrager nennen, denn seine iiberaus kraftigen Querfortsatze gehen plotz- 

 lich unter einem Winkel von 60 gegen die Medianlinie nach hinten, ragen die Halfte 



1 In this case, the sacrum of a young bird has been chosen for illustration. The transverse 

 processes of the avian vertebra referred to appear to be ossified from separate centers. 



