20 REPTILES 



clavicles, or collar-bones ; and also approximate to the rhyn- 

 chocephalian type in their system of abdominal ribs. A 

 marked difference from crocodiles is to be found in the circum- 

 stance that three elements of the pelvis (ilium, pubis, and 

 ischium) enter into the composition of the acetabular cavity for 

 the reception of the head of the thigh-bone, or femur — a 

 generalised feature they possess in common with dinosaurs. 

 Phytosaurus itself was a reptile of the dimensions of a good- 

 sized crocodile, with an enormously long snout, at the base of 

 which opened the nostrils. 



The belodonts are remarkably interesting reptiles, for in a 

 provisional genealogical tree published in the Zoological 

 Society's Proceedings, for 1904 they occupy the intermediate 

 position between rhynchocephalians and birds, being placed, 

 however, much closer to the former than to the latter group. As 

 a matter of fact, we know at present nothing in any way inter- 

 mediate between belodonts and birds, between which there is 

 evidently a very long gap; and we are consequently totally 

 ignorant of the mode of evolution of the many peculiarities in a 

 bird's skeleton. Not that this affords any argument against 

 the evolution of birds from reptiles, any more than does the 

 fact that we are ignorant of the direct ancestors of the ptero- 

 dactyles, and consequently the mode of evolution of the skeleton 

 of their wings. 



There are several notable features in the skull of the belo- 

 donts, in addition to the backward position of the opening of the 

 posterior nostrils. In the first place, the comparatively small 

 size of the apertures in the temporal region is a primitive feature, 

 very different from what obtains in the crocodiles as shown in the 

 figure of the skull of MetriorJiynchus. A second feature is the 

 great length of the premaxilla, which is also in marked contrast 

 to what obtains in crocodiles. 



Reverting to the Parasuchia as a starting-point, we may 

 probably regard the ichthyosaurs (Ichthyopterygia) as a side 

 branch from this stock, in which specialisation has been con- 

 centrated on the needs of an aquatic existence. Ichthyosaurs 

 agree with belodonts in their complete clavicles, but their teeth 

 are implanted in grooves, and evidence of kinship with stego- 

 cephalian amphibians is manifested in the retention of a partial 

 roof to the temporal region of the skull. Another early side 



