S30 Miss Johnson, On the development of the Peloic Girdle [Feb. 26, 



From a consideration of the adult forms, Marsh 1 has compared 

 the pubis of birds to the ' post-pubis ' of some of the Dinosaurs 

 described by him, while he views the pectineal process of the pubis 

 as being homologous with the pubis of the same Dinosaurs and 

 with the pubis of Reptiles. 



In Laosaurus, the embryo bird and Ornithorhynchus (see figs. 3, 

 2, 4), the various pubes are so similar in their double nature and the 

 almost equal development of both branches, that I think there can 

 be no doubt that Marsh has described the homologies correctly in 

 these cases. In Reptiles, the homologies are not equally obvious. 

 In them also, however, we find a double pubis. In Chelonia its 

 two branches are well developed. Both point forwards, and one is 

 directed inwards towards the symphysis, while the other passes 

 outwards. The latter is called the processus lateralis pubis. It is 

 found again in Lizards, but here it is more variable. It is given 

 off from about the middle of the outer surface of the pubis. 

 Generally it is directed downwards and outwards, but sometimes 

 it points directly backwards, as in Cyclodus (see fig. 1). The 

 pubis itself is directed forwards and inwards to the symphysis. In 

 a few Lizards, and in Crocodiles, the processus lateralis is absent. 



In comparing the two parts of the pubis in Reptiles with those 

 in Dinosaurs, Birds and Mammals, there are two views which most 

 obviously present themselves. Either (1) the processus lateralis 

 pubis is the pectineal process of the pubis, and the so-called pubis 

 is homologous in all cases, or (2) the processus lateralis becomes 

 the pubis of the higher forms, and the reptilian pubis becomes the 

 pectineal process. It seems to me that the relations of the bones 

 in Dinosaurs prove the greater correctness of the latter view. 

 Marsh 2 divides the Dinosaurs into four classes, in two of which no 

 post-pubis is present, but the pubis passes forwards and inwards, 

 forming the symphysis, and thus evidently corresponding to the 

 reptilian pubis. In the other two classes, the pubis is divided into 

 two branches. The posterior of these (' post-pubis ' of Marsh) points 

 backwards and thus resembles the pubis of birds, while the an- 

 terior ('pubis' of Marsh) is directed forwards and inwards, and 

 thus, though it does not form a symphysis, may be compared to 

 the pubis of the other Dinosaurs, and consequently to the reptilian 

 pubis. 



From this point of view, then, we see the processus lateralis 

 pubis first assuming a posterior position in such lizards as Cyclodus, 

 and becoming more important in Dinosaurs, till in adult Birds and 

 Mammals it takes the place of a functional pubis, and in Mammals 

 goes so far as to form a symphysis with its fellow. Meanwhile the 



1 0. C. Marsh, "Principal Characters of American Jurassic Dinosaurs." Amcr. 

 Journal of Science, 1878 and 1879. 



2 0. C. Marsh, "Classification of Dinosaurs." Amer. Journal of Science, 1882. 



