PRIMITIVE REPTILES 657 



later age. In the same and a later paper^^ I expressed the con- 

 viction that: 



At least two distinct groups have been associated among what are 

 called Microsauria, and that one of them, with single-headed inter- 

 central ribs and intercentral chevrons, represented by Hylonomus, 

 Petrobates, Sauravus and Eosauravus must be dissociated into a group 

 more nearly allied to, possibly identical with, the Reptilia in a wide 

 sense, while the other, of which Urocordylus may be taken as a type, 

 may remain with the Amphibia. 



For the former group I accepted the name Microsauria; for the 

 latter, tentatively, Lepospondyli. In the same year Thevenin,^^ 

 in a further discussion of Sauravus, with the description of an 

 additional species, evidently reached a like conclusion, referring 

 the genus to the Microsauria, taking. Hylonomus Dawson as 

 the type of the order, and placing it among the Reptilia. How- 

 ever, Dr. Moodie informs me that Hylonomus Credner, upon 

 which most of our knowledge of the genus is based, is by no means 

 certainly proven to be identical with Hylonomus Dawson, the 

 real type of the Microsauria; that indeed the latter may not 

 belong in the same group with the former, a conclusion not 

 surprising, considering the different geological ages of the two. 

 If such be the case, and it will not be possible to answer the ques- 

 tion satisfactorily until more material referable with certainty 

 to Hylonomus lyelli Dawson has been attentively studied, the 

 terms, as I use them, are merely tentative. 



Unfortunately the differential classification of the two groups 

 is still chiefly dependent upon the skeletal structure aside from 

 the characters of the skull, and, until the latter is better known 

 than it is at present among the Microsauria (in the above sense) 

 we lack a very important means of discrimination. For the 

 present therefore I would distinguish the two groups, whether 

 the Microsauria be reptiles or not, as follows (for the present 

 I leave out of account the Diplocaulidae and Aistopodidae) : 



Lepospondyli (as typified by Urocordylus). Vertebrae bolo- 

 spondylous, notochordal; ribs elongate, the head not attached 



26 Journal of Geology, vol. 18, p. 599, Oct., 1910. 

 " Annales de paleontologie, 5, p. 43, 1910. 



