344 On the Evolution of the Vertebrata, [ April, 
called degradation to be seen in the contents of the orders of 
reptiles! Among tortoises may be cited the loss of the rib- 
heads and of one or two series of phalanges in the especially 
terrestrial family of the Testudinidae. The cases among the 
Lacertilia are the most remarkable. The entire families of the 
Pygopodide, the Aniellidz, the Anelytropide and the Dibamide 
are degraded from superior forms. In the Anguide, Teidz and 
Scincide we have series of forms whose steps are measured by 
the loss of a pair of limbs, or of from one to all the digits, and 
even to all the limbs. In some series the surangular bone is lost. 
„In others the eye diminishes in size, loses its lids, loses the folds 
of the epidermis which distinguish the cornea, and finally is en- 
tirely obscured by the thickening of the cornea and closure of 
the ophthalmic orifice in the true skin. Among the snakes a 
similar degradation of the organs of sight has taken place in the 
order of the Scolecophidia, which live under ground, and often in 
ants’ nests. The Tortricidae and Uropeltide are burrowing 
snakes which display some of the earlier stages of this process. 
One genus of the true snakes even (according to Günther) has 
the eyes obscured as completely as those of the inferior types 
above named (genus Typhlogeophis). 
VII. Tue Avian Line. 
The paleontology of the birds not being well known, our con- 
clusions respecting the character of their evolution must be very 
incomplete. A few lines of succession are, however, quite ob- 
vious, and some of them are clearly lines of progress, and others 
are lines of retrogression. The first bird we know at all com- 
pletely, is the celebrated Archeopteryx of the Solenhofen slates 
of the Jurassic period. In its elongate series of caudal vertebre 
and the persistent digits of the anterior limbs we have a clear 
indication of the process of change which has produced 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 Cretaceous beds of England and North America, 
some of which have biconcave vertebrz, and all probably, the 
American forms certainly, possessed teeth. This latter character 
was evidently speedily lost, and others more characteristic of the . 
subclass became the field of developmental change. The parts 
_ 'Such forms in the Lacertilia have been regarded as degradational by Lankester 
> . 
