PLEXUS AND SACRAL VERTEBPwE OF LIZARDS. 527 



dancy of both osseous and nervous structures compared with those of the existing Rep- 

 tilia. Could we dissect the different forms of Pterosauria and Dinosauria, these diffi- 

 culties 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 rela- 

 tions, in this respect, to be readily and easily solved. 



In Birds generally, we find that a greater or less number of the vertebrae which are 

 postaxial to those bearing elongate ribs, develop short parapophyses which abut against, 

 and ankylose with, the preaxiaily extending, preacetabular part of the ilium. Postaxial 

 to these come vertebrae (which have been called 1 lumbo-sacral) which develop no para- 

 pophyses at all, or only minute rudiments of such, a fossa (the acetabular or renal 

 fossa) resulting from their absence. 



Behind these, again, there are generally vertebras with elongated parapophyses abutting 

 against the ilium ; and the first one, two, or three of these have often a more or less 

 decidedly different aspect from those which postaxially succeed. These vertebrae, which 

 we will provisionally call, for distinction, "posterior parapoyhysial," abut against the 

 postacetabular part of the ilium in the Strutbionidae and in most other birds. In the 

 Merganser they are placed very much postaxiad to the acetabulum. In Otis they 

 are opposite the middle of the acetabulum, and they are even more preaxiad in Tetrao 

 tetrix and Gallus, while in the Peacock (Pavo) they abut against the preaxial part of 

 the acetabulum. Thus in birds these posterior parapophysial vertebras (whatever may be 

 their true nature) sometimes have a position relative to the acetabulum nearly approach- 

 ing that which is found in the sacral vertebrae of some mammals, though generally their 

 relation to it is that of the sacral vertebrae of Saurians. 



In a few birds none of the vertebrae postaxiad to the acetabulum develop parapophyses. 

 This is the case, e. g., in the Hornbill (Buceros), in which the most postaxiad vertebra, with 

 parapophyses abutting against the ilium, abuts against the front margin of the aceta- 

 bulum. In the Woodpecker (Pica) the position of the most postaxiad of these parapo- 

 physes is still more removed preaxiaily from the acetabulum. 



In some Parrots the most postaxial of these vertebrae with parapophyses is even more 

 preaxiad to the acetabulum than in Pica ; but in other Parrot-forms there are also one or 

 two vertebrae with parapophyses opposite the middle of the acetabulum, and separated 

 by an interval from the preacetabular vertebrae with parapophyses. 



Judging, then, from the skeleton alone, we should be inclined to consider these posterior 

 parapophysial vertebrae abutting, in most Birds, against the postacetabular part of the 

 ilium as the homologues of the lacertian sacral vertebrae which also abut against the 

 postacetabular part of the ilium. And just also as we should consider the sacral vertebrae 

 of Alligators and Tortoises, in spite of their more preaxial position with relation to the 

 acetabulum, as really homologous with the sacral vertebrae of Lizards, so we should con- 

 sider the more preaxiaily situated posterior parapophysial vertebrae of Otis, Tetrao, 

 Gallus, and Pavo as homologous with the postacetabular posterior parapophysial vertebrae 

 of the Struthionidae and of most birds. "What, then, is to be said as to the vertebrae of 



1 Trans. Zool. Soc. vol. s. p. 9. 



4a2 



