374 Mn. k. i. rococK on the exterxai, 



The genal tuft, too, is generally indistinguishable in the latter 

 genus, although in a young specimen I have detected both genal 

 and interramal tufts, the former consisting of two vibrissjiB close 

 to the hinder corner of the eye. 



In Ccdogenys and Basyprocta the ordinaiy vibrissas are long 

 and numerous, especially those of the genal tuft in Coelogenys, 

 and the interramal tuft is present. In Dolichotis the vibrissas 

 generally resemble those of I)asy2^'>'octa ; but in an example of 

 I), scdinicola the interramal vibrissae, present in a specimen of 

 B.patagonica, were absent. In Hydrochoeriis all the vibrissse are 

 short and slender, the interramal are absent and the genal are 

 set below the level of the eye. In the typical species of Cavia 

 {C. 2^orcellus), the common guinea-pig, and in C. aperea, the 

 mystacials, superciliaries, and genals are as in the Octodontidse, 

 Dolichotis, and others, although relatively somewhat shorter; and 

 the interramals appear to be absent. But in an example of 

 Galea littoralis Thos.*, the interramals are represented by four 

 long bristles arranged along the posterior border of a neai'ly 

 naked area behind the chin, an arrangement recalling that of Ihe 

 ruminant ungulate Tragulus. 



In Chinchilla and Lagidium the mystacial vibrissse are exceed- 

 ingly long and coarse, but the superciliaries are much finer and 

 softer. The genals also, when present, are fine and soft, but I 

 found them in only one example of Chinchilla. In another 

 example of that genus and in a specimen of Lagidium I could 

 not detect them. In Lagostomus the mystacials, supei'ciliaries, 

 and genals are ail long and coarse, and this genus in addition 

 has a long thick mat of bristles on the cheek below the eye. 

 The interramals appear to be undeveloped in these three genera. 

 Ctenodactylus has long mystacials, superciliaries of medium 

 .length, but no discei'nible genals oi- interramals. 



The Mouth and Cheek-pouches. 



The mouth of the Hystricomorphs, as in other groups of 

 Rodents, is provided with lobes of skin jutting into it from the 

 cheek on each side and serving to shiTt ofl' the anterior from the 

 posterior part of the buccal cavity so as to prevent gnawed 

 fragments of wood from passing into the throat. A pair of 

 these, one on each side of the tongue, capable of meeting behind 

 the lower incisor, may be called the lingual lobes, while an upper 

 pair, capable of meeting across the palate behind the upper 

 incisors, may be called the palatal lobes. 



The lingual lobes, so far as my observations extend, always 

 retain their distinctness and can be separated or brought togethei'; 

 but the palatal lobes are more variable. Typically and in most 



* For the genera of tliis group, see Tliomas's paper, Ann. & Mag. Nat. Hist. (8) 

 xviii. pp. 301-303 (1916). I am indebted to Mr. Tliomas for tlie correct names of 

 the species of Cavies recorded in this paper. The specimen above referred to as 

 Galea littoralis is the cue I erroneouslj- identified as Cavia rvfescens when I 

 described its facial \'ibrissa3 in P. Z. S. 1914, pp. 900 and 905. 



