THE AFFINITIES OF BIRDS 165 



is generally regarded as the homologue of the pubis of birds, 

 while the forwardly directed half of the bone is considered 

 to be the equivalent of the pectineal process. The alterna- 

 tive is to look upon the prepubisof the dinosaur as the pubis 

 of birds, and the postpubis as having disappeared altogether 

 in them. In this case the crocodile will be an intermediate 

 form ; for in this reptile the prepubis is the ' pubis,' while 

 the postpubis is represented by an inconspicuous process 

 upon the pubis. The former alternative commends itself to 

 us. There are birds in which the pectineal process is 

 practically absent ; in others, as Apteryx, Geococcyx, &c., it is 

 large ; on the other hand there are dinosaurs, e.g. Laosaurus 

 censors, in which the prepubis is generally reduced — to a 

 length of not more than one-third of the postpubis (pubis 

 of birds). The opposite extreme is reached by Triceratops, 

 where it is the postpubis which is less than a third of the pre- 

 pubis, and Ceratosaurus, where it is even further reduced. 



It is not, however, only in the pubes that the pelvis of 

 the dinosaurs is like that of birds. In those reptiles the ilia 

 were extended forwards and backwards, as in birds, and in 

 Ceratosaurus at any rate the three bones were all firmly 

 ankylosed, as in all birds save ArchcBopteryx, where the 

 separation of the bones conforms to what is found in the 

 vast majority of the dinosaurs. Laosaurus, which has just 

 been mentioned, is one of the most birdlike of dinosaurs. 

 ' The two species of the genus first described by the writer,' 

 remarks Professor Mabsh, ' show these avian features best 

 of all, and it would be difficult to tell many of the isolated 

 remains from those of birds.' Of the cretaceous dinosaurs 

 the same author observes, ' Others were diminutive in size, 

 and so birdlike in form and structure that their remains 

 can be distinguished with difficulty, if at all, from those of 

 birds.' Ornithomimus, as its name denotes, is one of those 

 especially annectent dinosaurs. In this genus the third 

 metatarsal is crowded backwards behind the second and 

 fourth, as in many birds. But the dinosaurian metatarsals 

 which are most strikingly like those of birds are of Cerato- 

 saurus, which Marsh has figured side by side with those 



