380 MR. R. I. roCOCK ON THE EXTEKNAL 



anterior part of the wall of the subcircular cavity of the ear con- 

 taining the large exposed anrlitory orifice, the tragus, antiti'agus, 

 and the intervening notch being ol)literated. The antero-internal 

 ridge, forming the continuation of the strongly overfokled an- 

 terior rim of the ear, gradually disappears inferioily at about the 

 level of the orifice and does not curve shai-ply backwards beneath 

 the supratragus, (Text-fig. 6, M.) 



In Ccelogenys this ridge beneath the supratragus is much more 

 sharply defined and forms the lower edge of a well-defined depres- 

 sion, of which the supratragus, which has a definite thickening, 

 is the upper border, and above the supratragus there is another 

 weaker I'idge. There is a small tragus, but the antitragus is very 

 large and thick, and is marked above and posteriorly by a small 

 shallow pouch. Above this pouch the antitragus is continuous 

 with a ridge curving foi'wards above and forming the posterior 

 rim of the cavity of the eai'. The laminate portion of the eai- is 

 small, but erect, with a slight emargination near the middle of its 

 posterior border, a rounded summit, and an overturned anterior 

 margin continuous below with the aiitero-internal ridge. (Text- 

 fig. 7, D.) 



The ear of Dasyprocta is wider as compared with its height 

 than in Ccelogenys. As in that genus the inferior end of the 

 overturned anterior rim juts into the cavity, forming a horizontal 

 shelf-like ridge beneath the supratragus, which is itself a long 

 narrow ridge with a well-defined depression above it. There is a 

 small tragus as in Ccelogenys, but the antitragus is much less 

 developed than in that genus and has no pouch above it. Above 

 the tragus there is a little curled thickening, (Text-fig. 6, F.) 



In Cavia and allied genera the ear resembles tolerably closely 

 in shape that of Dasyprocta, although varying within the limits 

 of the genus. But it always dift'ers in having the supratragus 

 converted into a laminate valvular flap. In Galea littoralis the 

 ridge beneath the supratragus, which is also present in Dasyprocta 

 and Ccelogenys, is quite bhin and of uniform width throughout ; 

 the tragus is distinctly defined, and above it there is a thickening 

 recalling that seen in Dasyprocta but larger ; the antitragus, too, 

 is well developed, and the portion of the pinna below and behind 

 it is comparatively wide, wider than in Dasyprocta. But in Cavia 

 aperea and the Common Guinea-pig {C. porcellus) the ridge 

 beneath the supratragus is large and valvular, like the supra- 

 tragus itself, there is scaxcely a trace of tragal and antitragal 

 tliickenings, and the pinna is narrower behind the antitragal 

 ridge. When the ear of the Guinea-pig is folded the two valves 

 close down over the orifice, the supratragal valve lying upper- 

 most. The ear of Kerodon resembles that of the two last- 

 mentioned species of Cavia, although the two valves are relatively 

 a little smaller and the tragus and antitragus are a little better 

 developed. (Text-fig. 6, G, H.) 



The ear of Thrynomys, like that of Cavia, has the supratragus 

 valvular and the antero-internal ridge passing beneath it. There 



