OF THE MAMMALIAN OSSICULA AUDITUS. 383 



IV. In Colobus (and Cercocebus) the head of the malleus is almost as ill-developed 

 and sometimes more flattened than in Macacus, and the incus is of the same form. 



V. In Macacus, Cynopithecus, and Cynocephalus the malleus has a very short and 

 constricted neck; the manubrium, which bears a processus brevis and muscularis, forms 

 a very wide angle with the rest of the bone ; the incus and stapes exhibit constantly 

 the tendency to low types, already seen occasionally in higher Quadrumana. 



The Ossicula op the Cebid^:. 



The ossicula of the Spider-monkeys {Ateles) present some singular peculiarities not 

 observed in neighbouring genera. 



The head of the malleus (PI. LVIII. fig. 13) is extremely flattened vertically as well as 

 laterally ; seen from above it appears as a smooth, convex, almost quadrilateral surface. 

 The articular area is very wide, far more extensive than in any higher monkey ; the 

 upper segment or facet is not so wide as the lower, and they meet each other at a very 

 open angle without a trace of a groove. The upper lies almost vertically above the lower 

 segment, so that the whole surface resembles the condition seen in the lower Mammalia, 

 and is unlike the type of Sotno. Anteriorly a little friable layer of bone fills up a 

 part of the angle between the head and the processus gracilis, the stump of which is 

 stout. This homologue of the lamina seen in some mammals is often broken off and 

 absorbed in the mallei of old Spider-monkeys. The head never bulges prominently 

 forwards. 



So far the malleus bears characters common to the other Cebidse ; but it differs from 

 them all in possessing a singular neck, quite unlike that of the Anthropomorpha, though 

 almost as long. Through the kindness of Prof. Garrod I have been enabled to examine 

 numerous skulls of Ateles melanochir, A. hybridus, and A. ater, from specimens once 

 living in the Zoological Gardens; and I find these characters best marked in A. mela- 

 nochir. Immediately below the articular surface the neck is very stout, and it becomes 

 narrower close to the handle, though even there it is much less constricted than in the 

 apes and Old-world monkeys. The very short and curious manubrium forms with the 

 neck an angle often not more than 120°, though occasionally much wider. Constricted 

 near the base and tip, it bulges in a conspicuous manner in the middle, and terminates 

 in a wide discoidal enlargement. The processus brevis is represented by a prominent 

 rounded knob. There is no processus muscularis ; and the tendon of the tensor tympani 

 muscle is inserted into the root of the manubrium close to the neck. This malleus is 

 certainly the most specialized among all the Quadrumana. 



The incus has a very well-developed and very square body; in a specimen from A. hybridus 

 I find it slightly longer horizontally than vertically. The anterior segment is almost as 

 high as the posterior ; and the articular surface, looking rather outwards, appears unusally 

 deep from the outer surface of the body, and unusually shallow from the inner aspect. 

 The incus, on this account, will be seen to be most like that of Cebus, except in the last 

 feature, not showing that tendency to great height and narrowness so frequent amongst 

 other American monkeys. 



The stapes has a well-developed head, and long slender crura, not very divergent, and 



3p 2 



