CHARACTERS OF SOME HYSTRICOMORPH RODENTS. 377 



upper border. In both genera the tragus is distinctly developed, 

 and there is a large antitragal thickening. In Hystrix the 

 antero-internal ridge descends internally to the tragus ; in Athe- 

 rura it is a rounded lobate thickening above the tragus. The 

 supratragus is a narrow ridge concealed in front by the antero- 

 internal ridge and ending posterioi-ly in a ridge which descends 

 into the cavity of the ear. Behind the antitragus there is a long 

 deepish groove, and behind this the posterior edge of the ear 

 forms a flap capable of being folded forwards. (Text-fig. 1, A, E.) 

 The ear of Dinoinys is simple, with a ridge-like supi-atragus 

 and a thick, but not prominent antitragus. (Text-fig. 6, E.) 



The ear in Erethizon is covered thickly with woolly hair and 

 surrounded with spines. It is quite simple in structure and has 

 a tolerably evenly convex edge, which anteriorly above is curled 

 over and overlaps the anterior end of the simple supratragal 

 ridge, which posteriorly curves downwards and runs into the 

 tolerably large antitragal thickening. The lower margin of the 

 ear beneath the "aditus inferior" is a simple shallow ridge, and 

 the auditory orifice is not concealed by a tragal lobe, and there is 

 no definitely developed antero-internal ridge. (Text-fig. 6, C.) 



In Coendu the ear is of a much more specialised type. Its 

 outline is irregularly angular. The antitragal portion is exces- 

 sively develoj)ed into a high valvular thickening, and the con- 

 siderably overturned antero-superior edge is jointed at the level 

 of the supratragal ridge, so tliat the upper portion of the ear can 

 be folded down upon the lower portion, reducing the size of the 

 cavity, which can be similarly compressed from behind by the 

 forward movement of the posterior portion. Modified as the ear 

 is, it is clearly derivable from the type seen in Erethizon ; and, as 

 in the latter, the auditory orifice is not protected by a tragal 

 lobe. (Text-fig. 6, D.) 



The ear of Capromys is tolerably similar to that of Hystrix, 

 but not so expanded at the summit. The antero-internal ridge 

 descends to the auditory orifice and is continued superiorly a little 

 above the point of attachment of the pinna, but the overfolded 

 portion of the edge of the ear is here much narrower than in 

 Hystrix. There is a small tragus concealing the orifice, and a 

 narrow notch betwen it and the antitragal ridge which is also 

 less well developed than in Hystrix, and above and behind the 

 antitragus there is a small depression much shorter and shallower 

 than in the latter genus. As in Hystrix the simple supratragus 

 is overlapped in front by the antero-internal ridge, (Text- 

 fig. 2, A.) 



In Myocastor the ear is lower and rounder than in Capromys, 

 and has a laminate bulge on its posterior border above and 

 behind the antitragus. The antitragus is better developed than 

 in Capromys, and is provided with a large tuft of hairs which 

 serves to keep water out of the auditory orifice, which similarly 

 has a crown of short hairs above it to serve the same purpose. 

 The antero-internal ridge, instead of descending nearly vei^tically 



