86 



THE TROSIMIANS. 



The Aye-aye. 



The Aye-aye {C/iiromys tnadagascarieusis), 

 fig. 30, one of the rarest animals of the large 

 African island, is a species so peculiar as to 

 form by itself an entire family. On a first 

 hasty glance its large bushy tail and the 

 whole form of its body cause it to resemble 

 a squirrel of about the size of a cat. The 

 head is roundish, the snout but little pro- 

 truding, the ears large and naked, the eyes 

 large with round and v^ery contracted pupil, 

 the nose naked with sickle-shaped nostrils, 

 the lips provided with long moustache hairs, 

 the fur, which Is mostly of a grayish-brown 

 colour, mingled with long bristly hairs, which 

 are specially well developed on the tail. 



The adult animal has in each half of the 

 jaw, above and below, a strong sharp chisel- 

 shaped incisor, which is covered in front with 

 a very thick, behind with a very thin layer of 

 enamel, and being without a root reaches back 

 in the jaw to beyond the molars, and keeps 

 constantly growing as it is worn away by use, 

 a tooth accordingly which in all these respects 

 resembles that of the rodents. By their 

 action on one another these incisors get so 

 ground down that their front part projects 

 like a chisel while a broad heel or process is 

 formed behind. After these incisors there 

 follows, as in the rodents, a large gap or dias- 

 tema, and then in the upper jaw a small pre- 

 molar, lying very close to the three molars, while 

 in the lower jaw this premolar is wanting. 

 All the molars are simple, round, smcoth- 

 polished stumps. The dental formula is thus 



- — = 18 teeth, and the dentition of 



the adult is decidedly rodent in character. 



Altogether different is the milk-dentition, 

 which has been made known by Peters from 

 the examination of a new-born specimen, as 

 also of one a little older in which the per- 

 manent teeth were just on the point of cutting 

 the gum. From this examination it was found 

 that in each jaw, both above and below, there 



exists a second incisor, which afterwards drops 

 out, that in the upper jaw there is a canine 

 which is also ultimately lost, that both above 

 and below these appear two premolars, which 

 on the change of teeth disappear entirely 

 below, while the hinder of the two is replaced 

 in the upper jaw by a permanent tooth. The 

 milk-dentition accordingly comprises exactly 

 the same number of teeth as the permanent, 

 since in place of the three permanent molars 

 of the upper jaw there are one incisor, one 

 canine, and one premolar, and in place of 

 those of the lower jaw one incisor and two 

 premolars. The incisors of the milk-dentition 

 are divided into lobes and notched. 



That an animal with a dentition of this 

 nature cannot be regarded as belonging to 

 the rodents or gnawing animals, but must 

 be ranked with the prosimians, is beyond 

 question. But through the alteration of the 

 dental system above described the aye-aye 

 becomes one of the most remarkable animals 

 in creation. The inherited prosimian den- 

 tition, indicated in the dental formula of the 

 milk-teeth, has, by adaptation to an altered 

 mode of life, become converted into that of a 

 rodent through the disappearance of char- 

 acteristic elements. 



The hands and feet are formed in a highly 

 peculiar manner. Only the great toes have 

 a flat nail, all the other 4:oes and all the 

 fingers having claw-like arched nails. The 

 great toe is opposable and very powerful, but 

 the thumb, on the other hand, is small, and 

 has the same direction as the other fingers. 

 The fourth finger is inordinately long, the 

 fourth digit of the feet not so much so; the 

 middle finger is extraordinarily thin, the first 

 joint (phalanx) very long and curved, the 

 small phalanx carrying the claw very short, 

 so that the finger rather looks like a jointed 

 awl than a true finger. 



The aye-aye is the most persistent day- 

 sleeper known. Even a good shaking cannot 

 rouse the creature from its profound slumbers. 

 Only one specimen has hitherto been brought 



