MAMMALS THAT GNAW. 



(including tlie West Indies), the whole of the six families being represented 

 there, while four are confined to these countries, and one is luet with else- 

 where only in Africa. 



By far the largest of these families is that of the Octodontidm, which has 

 some representatives in Africa, although the majority of the forms are con- 

 fined to South and Central America and the West Indies. All have complete 

 collar-bones ; the vacuities in the front of the bony palate of the skull are 

 greatly elongated ; the crowns of the molar teeth are marked by infoldings 

 of enamel on both sides ; there are generally five toes to each foot ; and 

 the teats are placed liigh up on the sides of the body. In form, these 

 Rodents are usually more or less rat-like, and most are terrestrial in their 

 habits. 



The first group of the family is African, and is represented by two genera, 

 both easily distinguished by having the two inner toes of the hind-foot fur- 

 nished with a horny comb and bristles for the purpose of cleansing the fur. 

 Each genus has but a single species. The gundi, Ctenodactylus, is an inhabi- 

 tant of North Africa, near Tripoli, and is peculiar among the family in having 

 no premolars, the tail being reduced to a stump ; whereas in Pectinator, of 

 Somaliland, premolars are present, and the tail is moderately long and bushy. 

 Both are about the size of a water-vole. 



The second group comprises six genera, five of which are South American, 

 and the other African. They have partially rooted or rootless molars, with 

 simple enamel-folds and soft fur. The typical form is the degu (Octodmi) of 

 Chili and Peru, which is a rat-like animal, with a rather long'brush-tipped 

 tail, medium-sized ears, and the upper and lower molars alike. Other species 

 of the same genus inhabit Bolivia, which is also the home of the two species 

 of Habrocoma, characterised by the lower molars being more complex than 

 the upper, the large ears, and the extreme softness of the fur. Nearly 

 related are the burrowing tuco-tucos {Ctcnomys) of South America, deriving 

 tlieir popular name from the bell-like ciy uttered underground. They have 

 broad incisors, kidney-shaped crowns to the rootless molars, small eyes, 

 moderate ears, long claws, and bristles on the hind-feet. The one species of 

 the genus Aconcumys, from the Southern Andes, differs from the last by its 

 larger ears and shorter claws, and is further characterised by the enamel- 

 folds of the upper molars meeting in the middle. 



From Ctenomys, the two Chilian species of Sjjalacopiis diSev by their rudi- 

 mental ears ; they are noticeable on account of their laying up a winter store 

 of food. The very similar African genus Petromys^ may be distinguished 

 from the last by its harsher fur, the shortness of the first toe of the fore- 

 foot, and more thickly-haired tail. 



The coypu, or nutria (^Myopotainus), of South America, which is the only 

 member of its genus, is the typical representative of the last sub-family, 

 which has one African and ten American genera. In this group the molars, 

 which may be rootless or partially rooted, have deep and curved folds of 

 enamel, more or less harsh fur, which may be mingled with spines, and 

 the tail generally long. As a genus, the coypu, which attains a length 

 of about two' feet, has very large red incisors, two inner and two outer 

 enamel-folds in the upper molars, and three inner and one outer in the lower 

 ones, moderate-sized ears, a rather long, scaly, and sparsely haired tail, and 

 webbed hind-feet. Mr. Aplin writes that in tjruguay " it inhabits the larger 

 permanent lagunas. I have heard it stated that if a laguna is inliabited by 

 1 Some writers place this genus in the first sub-family. 



