592 
PROCEEDINGS OF THE NATIONAL MUSEUM. 
VOL, JXllJ, 
scapula?. Its skull and teeth show resemblances to those of Atherurm^ 
and the sacra in the two genera are practically identical. Atherurm , 
although showing strong affinities to Trichys, appears in certain ways 
to l>e related to Tkemrux, Most of the skeleton of Atherurm is much 
like that of Theeurm, the only striking difference being in the lessened 
number of sacral and caudal vertebra? in Atherurua , and in the pecul¬ 
iar axis of Thecurux, which does not resemble in any way the axis of 
any of the other three genera. The caudal bristles of Atherurm might 
have been derived from those of Tkecu/rm, or the reverse. If the cau¬ 
dal bristles of Atherurua had but one enlargement, and that more 
inflated and less flattened, they would he of the type found in Theeurux* 
Theeurnn differs from any of the other three genera by its peculiar axis. 
Without its skull and axis it could not be differentiated from Aeanthion, 
while if only its skull were known there would he little excuse for 
separating it from Atfu>ritru&, provided no account were taken of its 
rootless molars. Aeanthion is clearly closely related to Hystrir and 
less different from that genus than it is from Thecurua , Atherurua, or 
Trivhys. The five genera of Old World porcupines may be arranged 
serially thus; 
TLjatrix * Aeanthion * * Themrux - Atherurm * * * * Trichyg, 
with the most different genera at the extremes of the line and the most 
closely related next to one another. A break occurs between Tkecarux 
and At/tef'urus so that two subfamilies may be recognized. Ilyxtrim 
and Aeanthion are evidently directly and closely related to one 
another, and Thecurax is certainly much closer to them than it is to the 
Atherurua- Tetchyx group. Whether Atherurm and Triehya arc directly 
related to eaeli other or are only distantly so related through a remote 
ancestry is difficult to say. The two subfamilies, Hystricime and 
Atherurime are scarcely of equal rank, the members of the former 
being much more homogeneous than those of the latter. Tricky 8A 
with its generalized structure is evidently the most primitive of the 
Hystricida? and at the opposite end stands Hystrie (Plates LIV, LV, 
and LVI, fig. 4), the most specialized, with its peculiar much modified 
skull and highly developed quills. 
<*See Cederblotu, Zoot Jahrb,, XI, 1897-98, p, 513, and Wingt*, Jordfundne og 
nulevende tJnavere, Lagua Santa, Brasilien, 1887, pp. 12S, 129. 
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