78 



THE PROSIMIANS. 



in many cases so lax that the two halves fall 

 apart when one attempts to obtain a skeleton 

 of the animal. This character also suffices 

 to distinguish at the first glance the lower 

 jaw of a prosimian from that of a monkey. 



While the dentition of the Simia; presents 

 very constant numbers and relations, it is, on 

 the contrary, remarkably variable among the 

 Prosimii, and that both in respect of the 

 number of teeth and in respect of the structure 

 of the different sorts of teeth. In general 

 one may say that the dental system, especially 

 through the presence of double rows of sharp- 

 pointed cusps on the molars as well as 

 through the variations in the development of 

 the incisors and canines, is intimately allied 

 to that of the insectivores or marsupials, and 

 at any rate is altogether removed from that 

 of the monkeys. For the most part a more 

 or less considerable gap or diastema separates 

 the upper incisors in the same way as in the 

 bats, and the broad crowns of the incisors 

 are sometimes, as in Indris, placed sideways, 

 similarly to what we meet with in the 

 kangaroos. In other cases, as in Tarsius, 

 on the other hand, the hook -like curved 

 middle incisors of the upper jaw stand close 

 together, and far surpass in length the 

 canines as well as the crowns of all the other 

 teeth, while in the aye-aye the middle incisors, 

 which are the only ones that are present in 

 both upper and under jaw, entirely correspond 

 to those of the rodents in structure and in the 

 absence of roots. The incisors of the lower 

 jaw are always pressed close together in the 

 middle, are frequently six in number, and are 

 more or less flattened on the sides ; sometimes 

 (Lemur, Indris) they are placed horizontally 

 as in the kangaroos, while in Galeopithecus 

 they are even expanded and split into strips, 

 as in the rat-kangaroo (Hypsiprymnus). — Ca- 

 nines are altogether wanting in the adult aye- 

 aye, being only found (in the upper jaw) in 

 the milk-dentition. In Tarsius they are small 

 and insignificant; in others (Galago, Loris, 

 Microcebus) they are more prominent and 



stronger, and hook-like with a cutting edge; 

 in the true lemurs and in the genus Indris 

 they are triangular with a cutting edge, and 

 in Galeopithecus the first premolar, as in 

 many Insectivora, plays the part of a canine 

 as regards form and position. — The premolars, 

 which are present in the milk-dentition in the 

 aye-aye, are absent in the permanent dentition, 

 and its few molars are simple stumps with a 

 smooth surface without enamel folds or cusps, 

 as in many rodents. Among most of the 

 others the premolars form a sharp point, but 

 through the development of an inner tubercle 

 as well as through the division of the sharp 

 point gradually pass into the form of per- 

 manent molars, which mostly possess four 

 sharp cusps both in the upper and under jaw, 

 these cusps standing opposite one another in 

 pairs and interlocking when the mouth is 

 shut. True diastemas occur in the aye-aye 

 and in those genera (Lemur, Indris) in which 

 the canine in front is developed in breadth ; 

 in the other cases the crowns of the teeth 

 meet and merely present indentations opposite 

 the projecting cusps of the teeth of the 

 opposite jaw. 



The dentition constitutes beyond doubt the 

 most conservative element of the skeleton. 

 Constant relations in this part of the structure 

 bear witness to a common origin. But, as 

 may be seen from the preceding sketch, 

 no general scheme of the dentition of the 

 Prosimii can be drawn up; a proof that this 

 is only an apparent group of animals hetero- 

 geneously composed of the descendants of 

 various stocks, the types represented in 

 which approach one another only in the form 

 of their limbs. 



The brain has not the least resemblance to 

 that of the apes and monkeys. While the 

 smooth cerebrum of the small Clawed Mon- 

 keys still covers the cerebellum completely, 

 the latter is almost entirely free in the Prosimii, 

 and the few fissures or folds which the cere- 

 brum shows do not agree in their arrangement 

 with the general plan of structure seen in the 



