VERTEBRATA 55 



AAA. Scapular arch internal to ribs; sternum extend- 

 ing below coracoids and pelvis; one post- 

 orbital bar. 



No supramastoid ; a paroccipital ; clav- 

 icle not articulating with scapula ; (3) Testudinata. 



AAAA. Scapular arch external to ribs ; one longitu- 

 dinal postorbital bar (Synaptosauria). 



A supramastoid and paroccipital bones ; 

 ribs two-headed on centrum ; car- 

 pals and tarsals not distinct in form 

 from metapodials ; vertebrse amphi- 

 ccelous ; (4) Icthyopterygia. 



A supramastoid ; paroccipital not dis- 

 tinct ; a postorbito-squamosal arch ; 

 ribs two-headed ; a clavicle ; obtu- 

 rator foramen small or none ; ver- 

 tebrae amphicoelous ; (5) Theromora. 



* No supramastoid ; paroccipital not dis- 

 tinct; a quadra to-jugal arch; scapu- 

 la triradiate ; no clavicle ; ribs one- 

 headed ; (6) Plesiosauria. 



AAAAA. Scapular arch external to ribs ; two longitu- 

 dinal postorbital bars (paroccipital arch 

 distinct). (Archosauria). 



a, A supramastoid bone. 



Ribs two-headed ; a clavicle and inter- 

 clavicle ; acetabulum closed ; no 

 obturator foramen ; ambulatory ; 

 vertebrse amphicoelous ; (7) Pelycosauria. 



aa, No supramastoid. 



*Cope, Proc. Amer. Philos. Soc, 189J, identifies and describes a supramastoid in 

 tlie plesiosaurian skull. — Ed. 



