CHARACTERS OF SOME HYSTRIC05TORPH RODENTS. 423 



specialised descendants of forms akin, or belonging, to the Octo- 

 dontidfe, \yheveas the Hystricidie do not seem to be specially 

 related to any of the South-American groups. In that case the 

 resemblances between the Porcupines of the Old and New Worlds 

 which have led to their affiliation must be due to convergence oi' 

 to the common inheritance of ancestral characters. Jn their 

 spine- armature, for instance, it may be noted that in the two 

 genera which appear to rae to be the most primitive of the tuo 

 families' respectively — namely, Chceiomys and Trichys — the spines 

 are little more than very stiff bristles. It must be remembered, 

 however, that there is one character, not previously recorded 

 apparently, in which the two families are alike and differ from 

 other Hystricomorphs — the prepuce in the female does not form 

 a closed tube, the orilice of the urethra being exposed beneath 

 the genital aperture. 



Families Octodontid.e, Petromyid^, and Ctenodactylid^. 



I have seen too few examples of the Octodontidfe to offer any 

 opinion as to its subdivision into subfamilies. Petromys, too, I 

 have not seen, and I do not know whether it should be referred 

 to the Octodontidae, where it was originally placed, or, in accord- 

 ance with TuUberg's views, made the type of a special family. 

 Probably the latter is the better way of regarding it for the 

 present, in view of the differences of opinion that prevail concern- 

 ing its status. The structure of the ear alone seems to me to 

 justify its reparation from Ctenodactylus, with which Thomas 

 associated it. The claims of Ctenodactylus^ indeed, to a place in 

 the Hystricomorphs seem to me to be more than questionable. 



Families Capromyid.'B and Myocastorid^, nov. 



The family-name Capromyidee may be restricted to Gapromys and 

 related genera, like Procapromys, Geocajyromys, and, 1 presume, 

 Plagiodontia. 



Myocastor, formerly associated with these genera, may, I think, 

 be regarded as representing a family by itself, Myocastoridse. 

 This course, however, merely amounts to giving greater systematic 

 value to the characters used by Tullberg when he established the 

 subfamily Myopotamini. 



FamJly THRYNOMYiDiE, nom. nov. 

 (=Aulacodidae of Tullberg). 



The genus Thrynomys, formerly classified with the Octodontidje, 

 and later with the Capromyidee, was separated as the represen- 

 tative of a special family Aulacodidae by Tullberg, who employed 

 its old name Aulacodus. Agreeing with this decision, I adopt 

 Thrynomyidse as the family-title. 



