EVOLUTION OF THE VERTEBRATA. 335 



In the first jDlace, this line departs with lapse of time from the 

 primitive and ancestral order, the Theromorpha, in two respects : 

 First, in the loss of the capitular articulation of the ribs, and, sec- 

 ond, in the gradual elongation and final freedom of the suspensory 

 bone of the lower jaw (the os quadratum). In so departing from 

 the Theromorpha, it also departs from the mammalian type. The 

 ribs assume the less perfect kind of attachment which the mam- 

 mals only exhibit in some of the whales, and the articulation of 

 the lower jaw loses in strength, while it gains in extensibility, as 

 is seen in the development of the line of the eels among fishes. 

 The end of this series, the snakes, must therefore be said to be 

 the result of a process of creation by degeneration, and their lack 

 of scapular arch and fore limb and usual lack of pelvic arch and 

 hind limb, are confirmatory evidence of the truth of this view of 

 the case. 



Secondly, as regards the ossification of the anterior part of the 

 brain-case. This is deficient in some of the Theromorpha, the an- 

 cestral order, which resemble in this, as in many other things, 

 the contemporary Batrachia. Some of them, however (Diadecti- 

 dae), have the brain comjDletely inclosed in front. The late orders 

 mostly have the anterior walls membranous, but, in the strepto- 

 stylicate series at the end, the skull in the snakes becomes en- 

 tirely 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 sternum, no order 

 l^ossesses as many elements as thoroughly articulated for the use 

 of the anterior leg as the Permian Theromorpha, though the cor- 

 acoid is of reduced size. In all the orders there is loss of parts, 

 excepting only in the Ornithosauria and the Lacertilia. In the 

 former the adaptation is to flying. The latter retain nearly the 

 Theromorph type, enlarging the coracoid. An especial side de- 

 velopment is the modification of abdominal bones into two pecul- 

 iar elements to be united with the scapular arch into a plastron, 

 seen in the Testudinata. In this part of the skeleton the orders 

 are generally degenerate, the last one, the Ophidia, especially so. 



The pelvic arch has a more simple history. Again, in the 

 Theromorpha we have the nearest approach to the Mammalia. 

 The only other order which displays similar characters is the 

 Ornithosauria (Dimorphodon, according to Seeley). In the Dino- 

 sauria we have a side modification which is an adaptation to the 

 erect or bipedal mode of progression, the inferior bones being 



