70 THE GENEALOGY OF THE VERTEBRATA. 



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 ancestral order, which resemble in 

 this, as in many other things, the co temporary Batrachia. 

 Some of them, however (Diadectidaa), have the brain 

 completely enclosed in front. The late orders mostly 

 have the anterior walls membranous, but in the strepto- 

 stylicate series at the end, the skull becomes entirely 

 closed in front. In this respect then the snakes may be 

 said to be the highest or most perfect order. 



V. THE AVIAN LINE. 



The paleontology of the birds not being well known, 

 our conclusions respecting the character of their evolu- 

 tion must be very incomplete. A few lines of succession 

 are, however, quite obvious, and some of them are clearly 

 lines of progress, and others are lines of retrogression. 

 The first bird we know at all completely, is the cele- 

 brated Archeopteryx of the Solenhofen slates of the 

 Jurassic period. In its elongate series of caudal vertebrae 

 and the persistent digits of the anterior limbs, we have a 

 clear indication of the process of change which has pro- 

 duced the true birds, and we can see that it involves a 

 specialization of a very pronounced sort. The later 

 forms described by Seeley and Marsh from the Creta- 

 ceous beds of England and North America, some of which 

 have biconcave vertebrae, — all probably, the American 

 forms certainly, possessed teeth. This latter character 

 was evidently speedily lost, and others more character- 

 istic of the subclass became the field of developmental 

 change. The parts which subsequently attained especial 

 development are the wings and their appendages ; the 

 feet and their envelopes, and the vocal organs. Taking 

 all things into consideration the greatest sum of progress 



54 



