CHARACTERS OF SOMK HYSTIUCOMORPH RODENTS. 381 



is a small tragus and a moderately laroe antitragvxs. (Text- 

 fig. 7, A.) 



The ear of Dolichotis is much longer than in Cmna or 

 Coelogenys, and is somewhat narrowed above by the tolerably 

 deep and long emargi nation of its supero-posterior edge. The 

 supratragus is a simple ridge without the thickening seen in 

 Gcelogen.ijs. The continuation of the antero-internal ridge is very 

 distinct and long, longer than in Cvelogenys, but equally simple 

 in structiu'e. Beneath it there is a similar ridge curving abruptly 

 downwards to end in a thickening above the auditory orifice. 

 The tragus is quite well developed and high, but the antitragus 

 is not better developed than in Cavicc. (Text-fig. 7, C.) 



In Hydrochcerus the ear is very much simplified, the only 

 strongly developed ridge being the supratragus, which terminates 

 somewhat abruptly near the middle of the cavity. The antero- 

 internal ridge is not turned backwards beneath the supratragus, 

 but gradually fades away inferiorly. There is no definite tragus 

 or antitragal thickening, and the ridge representing the latter is 

 not continued so high as the posterior end of the supratragus. 

 The depression in which the orifice lies is thickly overgrown Avith 

 hairs covered with waxy secretion, the two combining to exclude 

 water. (Text-fig. 7, C.) 



The ear of Layostomus trichodactylus is very specialised, but 

 seems to be an extreme modification of some such type as that 

 seen in Coelogenys. Its laminate portion is very much larger 

 and is tolerably evenly oval. There is a v/ell-developed tragus 

 separated by a large notch from the antitragal portion, which, 

 however, does not form a thickened bulge as in Coelogenys, but is 

 continued obliquely upwards and backwards as a long straightish 

 sharp ridge defined behind by a shallow depression running in- 

 feriorly into a shallow pouch, doubtless homologous to the 

 similarly placed pouch above and behind the antitragus in 

 Coelogenys. The backwardly curved continuation of the a,ntero- 

 internal ridge is strong and forms the lower border of a deep 

 depression, bordered above by the thickening representing the 

 suprati'agus, but this does not form a definite ridge as in most of 

 the ears hitherto described. (Text-fig. 7, F.) 



Tiie ear of Chinchilla is tolerably similar in tj^pe to that of 

 Lagostomus, but is mvich more expanded, except a,t the base 

 where it is narrower and more tubular, so that the orifice lies 

 deeper in the ear at the bottom of the depression bordered abovo 

 by the continuation of the antero-internal ridge. No trace of 

 the supratragus remains. The tragus, too, has disappeared ; but 

 the a.ntitragus is developed into a prominence relatively as large 

 as in Coelogenys ; and this is hollowed into a deep pocket, from 

 the centre of which arises a straight ridge corresponding to the 

 similar straight lidge in Lagostomus and to the curved ridge 

 defining the cavity of the ear posteriorly in Coelogenys. (Text- 

 fig. 7, E.) 



Proo. Zool. Soc— 1922, No. XXVI 26 



