136 Annals of the Carnegie Museum. 



The postglenoid process is unusually heavy. In a specimen, No. 

 3201, referred to P. ohliquidens the post-tympanic and paroccipital 

 processes are separated, as shown in Plate XLVI, Fig. i, and answers 

 quite well to Cope's description. Furthermore the actual specimens 

 have been compared, and are found to be quite alike in this respect. 

 There are a number of similarities in the contour of the present speci- 

 men and the young specimen on which Cope established his Triplopns 

 c II bi talis. 



These similarities may be regarded as of comparatively little 

 importance since both specimens pertain to young animals. 



In comparing Prothyracodon with Doctor Koch's Prohyracodon^-^ 

 as described and illustrated in Doctor O. Abel's work^-^ it is at once 

 seen that M- and M^ of Prohyracodon, though mutilated, show the 

 proto- and metalophs to be at a more nearly right angle to the ectoloph, 

 the proto- and metalophs are also of more nearly subequal size than in 

 the American genus. Furthermore there is in Prothyracodon a 

 better defined antecrochet, a proportionally longer M-, and M- has a 

 more nearly triangular outline, than in the specimen preserved at 

 Budapest. 



M EASUREMENTS. 



Total length of skull from condyle to and including d. P- 165 mm. 



Antero-posterior diameter of deciduous upper dentition 42 



" M' and M- 36 



. " " " 18 



Transverse " " M- 16 



" M^ 19 



Antero-posterior " " " 22 



deciduous lower dentition 42 



permanent My and M2 32 



Mt 15 



Transverse 



" M2 •. 10 



Antero-posterior " " " 16 



On comparing the atlas of No. 3007 with that of Ilyracodon, the 

 similarities are remarkably close. Thus it is seen, that, as in the 

 latter genus, the bone is high^^^ and rather short, with a backward 



128 Koch, A., Termeszetrazi Fuzetek, Budapest, XX, 1897, pp. 490-500, Pis. 

 XII-XIII. 



^'^^ AbhancU. der K. K. Geolog. Reichsanslall, Bd. XX, 1910, pp. 24-25, PI. II, 

 Fig. I. 



131 The atlas of Prothyracodon appears to be proportionally higher than in 

 Hyracodon, which is to a certain extent due to crushing. 



