EVOLUTION OF THE VERTEBRATA. 337 



Leaving this order, we soon reach the prevalent ball-and-socket 

 type of the majority of Eeptilia. This strong kind of articula- 

 tion is a need which accompanies the more elongated column 

 which itself results at first from the posterior direction of the 

 ilium. In the order with the longest column, the Ophidia, a sec- 

 ond articulation, the zygosphen, is introduced. The mechanical 

 value of the later reptilian vertebral structure is obvious, and in 

 this respect the class may be said to present a higher or more 

 perfect condition than the Mammalia. 



In review it may be said of the reptilian line, that it exhibits 

 marked degeneracy in its skeletal structure since the Permian 

 epoch ; the exception to this statement being in the nature of the 

 articulations of the vertebrae. And this specialization is an adap- 

 tation to one of the conditions of degeneracy, viz., the weakening 

 and final loss of the limbs and the arches to which they are at- 

 tached. 



The history of the development of the brain in the Eeptilia 

 presents some interesting facts. In the Diadectid family of the 

 Permian Theromorpha it is smaller than in a Boa constrictor, 

 but larger than in some of the Jurassic Dinosauria. Marsh has 

 shown that some of the latter possess brains of relatively very 

 narrow hemispheres, so that in this organ those gigantic reptiles 

 were degenerate, while the existing streptostylicate orders have 

 advanced beyond their Permian ancestors. 



There are many remarkable cases of what may now be safely 

 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 

 Pygopodidae, the Aniellidae, the Anelytropidae, and the Dibamidae 

 are degraded from superior forms. In the Anguidae, Teidae, and 

 Scincidae, 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 distinguishes the cornea, and finally is en- 

 tirely obscured by the closure of the ophthalmic orifice in the 



* Such forms in the Lacertilia have been regarded as dcgradational by Lan- 

 kestcr and IJoulunger. 



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