68 THE GENEALOGY OF THE VERTEHRATA. 



A A. Elements of extremities differentiated. 



II. Os quadratum immovably articulated ; capitular and tubercular 



rib-articulations distinct. Arcbosauria. 

 Pubis and ischium united, and with little or no obturator foramen ; one 

 posterior cranial arch ; limbs ambulatory ; a procoracoid 



2. Theromorpha. 



Ischium and pubis distinct, the latter directed forwards, backwards or 



downwards ; two posterior cranial arches ; limbs ambulatory ; no 



procoracoid 3. Dinosauria. 1 



Ischium and pubis united ; two postcranial arches ; anterior limbs 

 volant 4. Omithosauria. 



III. Os quadratum closely united to cranial arches ; but one rib-ar- 



ticulation. Synaptosauria. 

 Distinct hyposternal and postabdominal bones ; ribs joining each two 

 vertebra?, and generally forming a carapace ; and 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 :; . 

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

 sistent .... 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 or sphenoid and presphenoid bones 



10. Ophidia. 



An inspection of the characters of these ten orders, 

 and their consideration in connection with their geologi- 

 cal 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 



1 This definition includes the Crocodilia in the Dinosauria, as it is absolutely connected 

 with the typical Dinosaurs by the Opisthoccela (Sauropoda Marsh). 



SEpisternum present. 



3 It is quite possible that the three divisions of this head form one natural order, the 

 Streptostylica, or Squamata. 



52 



