i2o PRIMARY FACTORS OF ORGANIC EVOLUTION. 



membranous, but, in the streptostylicate series at the 

 end, the skull in the snakes becomes entirely closed in 

 front. In this respect, then, the latter may be said to 

 be the highest or most perfect order. 



As regards the scapular arch, including the ster- 

 num, no order possesses as many elements as thor- 

 oughly articulated for the use of the anterior leg as the 

 Permian Theromora (excepting in the suborder Pelyco- 

 sauria). In all the orders there is loss of parts, except- 

 ing only in the Ornithosauria and the Lacertilia. In 

 the former the adaptation is to flying. The latter re- 

 tain nearly the theromorous type. An especial side 

 development is the modification of abdominal bones 

 into three pairs of peculiar elements to be united with 

 part of the scapular arch into a plastron, and the in- 

 clusion of the coracoid above them, seen in the Testu- 

 dinata. 



The pelvic arch has a more simple history. Again, 

 in the Theromora we have the nearest approach to the 

 Mammalia. The only other order which displays sim- 

 ilar characters is the Ornithosauria (Dimorphodon, 

 according to Seeley). In the Dinosauria we have a 

 side modification which is an adaptation to the erect 

 or bipedal mode of progression, the inferior bones be- 

 ing thrown backward so as to support the viscera in a 

 more posterior position in birds. This is an obvious 

 necessity to a bipedal animal where the vertebral col- 

 umn is not perpendicular. And it is from the Triassic 

 Dinosauria that I suppose the birds to have arisen. The 

 main line of the Reptilia, however, departs from both 

 the mammalian and the avian type and loses in strength 

 as compared with the former. In the latest orders, 

 the Pythonomorpha and Ophidia, the pelvis is rudi- 

 rnental or absent. 



