ii 4 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



No supramastoid ; sub- and postpelvic ossifications ; in- 



terclavicle and clavicles separated from and below 



scapular arch ; ribs one-headed on centrum ; coracoid 



large, free posteriorly ; Testudinata. 



aa. Paroccipital bone not distinct. 



Ribs one or two-headed, capitulum intercentral ; clav- 

 icles and interclavicles forming part of shoulder- 

 girdle ; scapula simple ; pubis and ischium plate-like 

 with small or no obturator foramen ; no sub- or 

 post-pelvic bones ; no supramastoid ; Theromora. 

 Supramastoid present ; ribs one-headed ; scapula trira- 

 diate ; no sternum ; pubis and ischium plate-like ; no 

 sub- or postpelvic bones ; Plesiosauria. 



AAA. Cranium with two postorbital bars ; a sternum. (Archo- 

 sauria. ) 

 Paroccipital bone not distinct ; no supramastoid. 



Ribs two-headed ; no interclavicle ; external anterior 

 digits greatly elongate to support a patagium ; 



Ornithosauria. 



Ribs two-headed ; no interclavicle ; acetabulum per- 

 forate ; limbs ambulatory ; Dinosauria. 

 Ribs two-headed ; an interclavicle ; acetabulum closed ; 

 feet ambulatory ; Crocodilia. 

 Ribs one headed ; an interclavicle ; acetabulum closed ; 

 feet ambulatory ; Rhynchocephalia. 

 II. The quadrate loosely articulated with the adjacent elements, 

 and only proximally. (Streptostylica. ) 



One postorbital bar, when present ; a paroccipital ; su- 

 pramastoid not distinct ; ribs one-headed; Squamata. 



An inspection of the characters of these ten orders, 

 and their consideration in connection with their geo- 

 logical history, will give a definite idea as to the char- 

 acter of their evolution. The history of the class, and 

 therefore the discussion of the question, is limited in 

 time to the period which has elapsed since the Per- 

 mian epoch inclusive, for it is then that the Reptilia 

 enter the field of our knowledge. During this period 

 two remarkable orders of reptiles inhabited the earth, 



