io6 



THE AM ERICA \ \A IL'RALIST. [Vol. XXXVIl I. 



bounded by the supra anil latcro-temporal arches, one or both of 

 which may secondarily disappear. In turther contrast with the 

 Synapsida, the squamosal is a relatively small element, fre- 

 quently separate from the prosquamosal, and never entering into 

 articulation with the lower jaw ; the quadrate, on the other hand, 

 is a relatively large element, uncovered, and sometimes second- 

 arily movable. In the shoulder girdle we find a most distinctive 

 character in the early coalescence of the coracoid and procoracoid 

 into a single bone, or in the degeneration of the procoracoid. 

 Another highly distinctive character is the phalangeal formula, 

 2- 3- 4- 5- 3-4» which is secondarily modified in the aquatic 



There appear to have been two great adaptive radiations 

 among the Diapsida. The first is that which occurred during 

 the upper Carboniferous and Permian, branches of which are 



already well separated in the Permian and have been collectively 

 grouped in the superorder Diaptosauria by Osborn. The 

 snond, or later radiation of the Diapsida, partly sprung from 

 terminal branches of the first, is known in the Triassic, and 

 inchulos the great orders Parasuchia, Ichthyopterygia, Croco- 

 chha. the superorder Dinosauria, the superorder Squamata, and 

 finiilly the Pterosauria. 



