658 S. W. WILLISTON 



intercentrally; chevrons exogenous processes from body of ver- 

 tebra; no intercentra; a single sacral vertebra; carpus and tarsus 

 unossified; or, if ossified, a separate intermedium tarsi present; 

 skull with two occipital condyles; parasphenoid well developed. 



Microsauria (as typified by Eosauravus, Sauravus, Hylonomus 

 Credner and Petrobates). Vertebrae holospondylous, notochor- 

 dal; ribs long, attached intercentrally, usually without distinct 

 separation into head and tubercle; chevrons intercentral, not ex- 

 ogenous processes from the centrum. One (?) or two sacral verte- 

 brae; carpus and tarsus ossified; no separate intermedium pedis; 

 phalangeal formula 2, 3, 4, 5, 4 in foot (Eosauravus, Sauravus). 

 Interclavicle with long posterior stem. Skull (imperfectly known 

 in Hylonomus Credner and Petrobates Credner) evidently stego- 

 crotaphic; palate with small teeth; parasphenoid slender (in front 

 at least). Small animals. 



Intercentra and dorsal ossifications have been discovered in 

 none of these genera while ventral ribs are known in all save 

 the single known specimen of Eosauravus copei, in which they 

 may have been lost in fossilization. Ossified mesopodials are 

 known in all the genera, and apparently all have but the two 

 bones, astragalus and calcaneum in the proximal row of the 

 tarsus. Hylonomus Credner and Petrobates are from the middle 

 RothHegende of Germany, Sauravus Thevenin typically is from 

 the uppermost carboniferous of France, and Eosauravus Williston 

 is from about the middle of the Pennsylvanian of Ohio. 



Taking these genera into consideration as a whole, it is seen 

 that there is nothing yet known to differentiate them from the 

 other primitive genera included under the definitions of the fore- 

 going pages (unless it be the absence of intercentra); while, on 

 the other hand, there are a number of characters, and those of con- 

 siderable importance, that distinguish them from the Amphibia. 

 It was some of these characters which induced Moodie to give 

 an ordinal position to the Diplocaulidae, in which he was quite 

 right, under the assumption that the Microsauria as a whole are 

 a homogeneous group. But it is evident that, as usually grouped 

 the Microsauria are not homogeneous; that some of the genera 

 hitherto included among them are genuine amphibians, in all 



