408 ALICE JOHNSON. 



form of which; therefore, may be accounted for in this way. 

 The pubis, being j)lace(l lowest, loses its functional importance 

 as a point of support for muscles and becomes very slender. Its 

 middle portion may even abort altogether, as sometimes happens 

 in ducks and other swimming birds. 



So far it appears quite possible to explain the facts by the 

 first theory. 



Turning to the second, we have to imagine a somewhat 

 different process. The processus lateralis of reptiles, in 

 becoming the pubis of the higher forms, has retained the posi- 

 tion which it had already come to occupy in some Lizards (see 

 fig. 14), and has increased in extent and functional importance. 

 In Mammals it goes so far as to form a new symphysis, while 

 in birds the backward direction of the bone is very much exag- 

 gerated. The part corresponding to the reptilian pubis at first 

 retains its original situation and almost its original dimensions, 

 as the anterior branch of the pubis in Dinosaurs, the embryo 

 bird, and Ornithorhynchus. It gradually dwindles into the 

 subordinate position of the pectineal process. 



This theory again, accounts for all the known facts, and it 

 agrees, better than does the former view, with the" relations of 

 the pubis in Dinosaurs. Marsh ^ found that the anterior 

 branch of the pubis in the Stegosauria and Ornithopoda, e. g. 

 Laosaurus, passed forwards and inwards, ending in a broad 

 spatulate free extremity. In the Theropoda and Sauropoda, 

 e. g. Atlantosaurus, no posterior branch of the pubis existed, 

 but the bone which evidently corresponded to the anterior 

 branch in Laosaurus formed the symphysis. Judging from 

 this fact there seems no doubt that the anterior branch is 

 homologous to the reptilian pubis. I think there can also be 

 no doubt of the homology between it and the anterior branch, 

 which, however, no longer forms the symphysis in birds and 

 mammals. 



These conclusions may be tabulated as follows : 



^ 0. C. Marsh, " Classification of Dinosaurs," ' American Journal of Science' 

 (Silliman), 1882. 



