PHYLOGENY. 123 



of the orders of reptiles.^ Among tortoises may be 

 cited the loss of one or two series of phalanges in sev- 

 eral especially terrestrial families of the Testudinidae. 

 The cases among the Lacertilia are the most remark- 

 able. The entire families of the Pygopodidae, the 

 Anniellidae, the Anelytropidse, and the Dibamidae are 

 degraded from superior forms. In the Anguidae, Te- 

 idae, and Scincidse, 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 entirely obscured by the closure of the oph- 

 thalmic orifice in the true skin.^ Among the snakes 

 a similar degradation of the organs of sight has taken 

 place in two suborders, which live underground, and 

 often in ants' nests. The Tortricidae and Uropeltidae 

 are burrowing-snakes which display some of the earlier 

 stages of this process. One genus of the true colubrine 

 snakes even (according to Giinther) has the eyes ob- 

 scured as completely as those of the inferior types 

 above named (genus Typhlogeophis.) 



e. The Avian Line. 



The paleontology of the birds not being well known, 

 our conclusions respecting the character of their evo- 

 lution must be very incomplete. A few lines of suc- 

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

 are clearly lines of progress, and others are lines of re- 



ISuch forms in the Lacertilia have been regarded as degradational by 

 Lankester and Boulanger. 



2A table of the degenerate forms of Lacertilia is given in the chapter on 

 Catagenesis. 



