MAMMARY GLANDS IN LISSENCEPHALA. 775 



in colour and appearance to the surrounding herbage : H the nest 

 is generally inhabited by the pair, with three or four young, when 

 these are so large as to quit the pouch. 



§ 415. In Lissencephala. — The fertile and commonly muci- 

 parous species of the Rodent order have corresponding provision 

 for the nourishment of the young in the number of nipples. A 

 seeming exception is presented by the domesticated breed of S. 

 American Cavy, called ' Guinea-pig.' The prolific power of this 

 well-fed pet is proverbial : they begin to breed at two months old, 

 and gestation may be repeated at intervals of two or three months. 

 The first litter consists of four, the second of five or six ; and, 

 as full maturity is gained by the mother, she may bring forth 

 eight, ten, and even twelve young : and yet she has only two 

 nipples to serve them, turn and turn about. The teats are large 

 and prominent, but lodged in a small shallow pouch, one on each 

 side of the hind part of the belly. The mammary glands, how- 

 ever, attain a size more in accordance with the demands upon 

 them : they are not pendant, like an udder, but flat and spread 

 over the abdomen. The wild original (Cavia aperea, Linn.) 

 breeds but once a year, and then has but one or two young. 

 Dolickotis has but two young. Echimys appears to have but 

 two teats, placed midway between fore and hind legs: Rennger 

 found two young in the nest at the bottom of the burrow : they 

 were blind The Paca ( Ccelogenys), with commonly two young, 

 has four teats ; and this number is not surpassed in Lagostomus, 

 Cercolabes, Erithizon, Hystrix, Capromys, Helamys, Dlpus, and 

 some species of Sciurus (Sc. palmar um). In the Biscacha the 

 anterior pair of teats is IJ inches behind the base of the fore- 

 legs : the posterior pair 2 inches farther back. In the prehensile 

 Porcupine the hind nipple is midway between the fore and hind . 

 leg, the front nipple midway between this and the base of the 

 fore-leg. Both these species, the common and the Canadian Por- 

 cupines, have usually two at a birth. The mother Coypu usually 

 carries her young upon the back in her frequent traverses of the 

 river she frequents : her teats are easily reached by the young so 

 transported, as they project from the flanks, nearer the back than 

 the belly : the anterior pair are just behind the shoulders : the 

 posterior pair anterior to the haunches. The nipples are rather less 

 elevated than in the Coypu, in Hydrochcerus, which swims with the 

 young on her back : they are also lateral in Lagostomus, Octodon, 

 Habrocoma, and Nelomys. 2 In Octodon the foremost nipple is 

 ^ inch behind the base of the fore-leg : the hindmost pair are 



1 ccxxxix". 2 cclxxx". p. 299. 



