592 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxii. 



scapula?. Its skull and teeth show resemblances to those of Atheruras^ 

 and the sacra in the two g-enera are practically identical, Atheriirus, 

 although showing strong affinities to Trichys, appears in certain ways 

 to be related to Thecurus. Most of the skeleton of AfherurusAH nuich 

 like that of Thecunis^ the only striking difference being in the lessened 

 number of sacral and caudal vertebrae in Atherurus^ and in the pecul- 

 iar axis of Thecurim^ which does not resemble in any way the axis of 

 any of the other three genera. The caudal bristles of Atherurus might 

 have been derived from those of Thecurus, or the reverse. If the cau- 

 dal bristles of Atherurus had but one enlargement, and that more 

 inflated and less flattened, they would be of the type found in Thecnrus. 

 Thecurus difi'ers from an}' of the other throe genera by its peculiar axis. 

 Without its skull and axis it could not be difl'erentiated from Acant/t ion, 

 while if only its skull were known there would be little excuse for 

 separating it from Atherurus, pro\'ided no account were taken of its 

 rootless molars. Acanthion is clearly closely related to Ilystrid' and 

 less difl'erent from that genus than it is from Thecurus, Atherurux, or 

 TricJiys. The live genera of Old World porcupines may be arranged 

 serially thus: 



Hystrix * Acanthi on ""' * Thecurus Atherurus * * * * Trlchys, 



with the most difl'erent genera at the extremes of the line and the most 

 closely related next to one another. A break occurs between Thecurus 

 and Atherurus so that two subfamilies may be recognized. Hystrix 

 and Acanthion are evidently directly and closely related to one 

 another, and Thecurus is certainly much closer to them than it is to the 

 Atherurus- Trichys group. W^hether Atherurus and Trlchys are directly 

 related to each other or are only distantly so related through a remote 

 ancestry is diflicult to say. The two subfamilies, Hystricinas and 

 Atherurinfe are scarcely of equal rank, the members of the former 

 being much more homogeneous than those of the latter. Trichys,^ 

 with its generalized structure is evidently the most primitive of the 

 Hystricidffi and at the opposite end stands Hystrix (Plates LIV, LV, 

 and LVI, flg. 4), the most specialized, with its peculiar much modified 

 skull and highly developed quills. 



«See Cederblom, Zool. Jahrb., XI, 1897-98, p. 513, and Winge, Jordfundne og 

 nulevende Gnavere, Lagoa Santa, Brasilien, 1887, pp. 128, 129. 



