334 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



Distinct hyposternal and postabdominal bones ; ribs joining each two vertebras, and 

 generally forming a carapace ; one posterior cranial arch 5. Testudinata. 



Hyposternal and postabdominal bones not distinct ; two posterior cranial arches ; 

 ribs attached to one vertebra ; a sternum ; ? no procoracoid 



6. Rhynchocephalia. 



Hyposternal and postabdominal bones not distinct ; two posterior cranial arches ; 

 ribs attached to one centrum ; no sternum ; * a procoracoid . . 7. Sauropterygia. 



IV. Os quadratum attached only at the proximal extremity, and more or less 

 movable; ribs with one head. Streptostylica.f 

 Brain-case membranous in front of prootic bone ; trabecula not persistent 



8. Lacertilia. 

 Brain-case with osseous walls anterior to prootic ; a scapular arch and sternum 



9. Pythonomorpha. 



Brain-case with osseous walls anterior to prootic ; no scapular arch nor sternum ; 



trabecular grooves of sphenoid and presphenoid bones 10. Ophidia. 



An inspection of the characters of these ten orders, and their 

 consideration in connection with their geological history, will give 

 a definite idea as to the character 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 Permian ei3och 

 inclusive, for it is then that the Keptilia enter the field of our 

 knowledge. During this period but one order of reptiles in- 

 habited the earth, so far as now known, that of the Theromori3ha. 

 The important character and role of this type may be inferred 

 from the fact that they are structurally nearer to both the Ba- 

 trachia and the Mammalia than any other, but present characters 

 which render it probable that all the other reptiles, with possibly 

 the exception of the Ichthyopterygia, derived their being from 

 them. The phylogeny may be thus expressed : 



Dinosauria Testudinata Rhynchocephalia Lacertilia Ophidia 

 (Crocodilia) 



Pterosaupia 



Pythonomorpha J 

 Sauropterygia^ 



Ichtliyopterygia Theromorpha 



* Episternum present. 



f It is quite possible that the three divisions of this head form one natural 

 order, the Streptostylica, or Squamata. 



X Some unknown type of this order will represent the ancestor of the Ophidia, 

 while the Lacertilia may have come directly from the Theromorpha. (Ed. 1886.) 



