OF THE MAMMALIAN OSSICULA AUDITUS. 471 



Thus the malleus of Bradypus has very general characters. It cannot he considered 

 a low type, like that of any marsupial ; hut its affinities are rather in the direction of 

 Mammals undeniahly higher than the Edentata, such as the higher Primates, the 

 Ichneumons, and the Horses, not from any positive characters, but rather from the 

 absence of salient modifications so prominent in animals much nearer to them in other 

 respects. 



The incus (PL LXIV. fig. 1) has a rather shallow body, with proportionally large 

 crura. The processus brevis is very stout and rather long, sharply pointed and very 

 divergent from the body ; the processus longus is well developed and broad. I have 

 not observed any bifurcation on its inner surface in three incudes from different 

 individuals in the College Museum, although Hyrtl names that peculiarity in describing 

 the incus of Bradypus. The Sylvian apophysis is small, but very distinct, being attached 

 to the crus by an extremely narrow pedicle. The affinities of this ossicle are very 

 uncertain ; the stout divergent processus brevis recalls the type of medium-sized Bovidse ; 

 but in the Sloth the incus is smaller in proportion to the malleus than is the case with 

 the Ungulata. 



The stapes in Bradypus tridactylus (fig. 1) is a very interesting bone, remarkable for 

 having occasional (not constant) features which relate it very decidedly to the Didelphia 

 and Sauropsida. In one pair belonging to the College series the head is moderately 

 developed, the crura are stout, rather long, and but little divergent, leaving between 

 them a narrow but very distinct aperture. In another pair in the same collection 

 the right stapes has no aperture, whilst the left has a minute foramen between its 

 crura. In all, the base is very broad, recalling its form and proportions in the Marsupial 

 stapes. 



Yet Hyrtl remarks {op. cit.), " It is most remarkable that Bradypus tridactylus has 

 only one crus, which is broad ; Bradypus didactylus (sic), on the other hand, has two 

 very thick crura, which, on that account are separated by a space bardly the width of a 

 hair. The simple crus of the stapes of B. tridactylus is very thin and transparent, whilst 

 the stapes in B. didactylus is massive and conical." He then remarks that he has only 

 examined the stapes of B. tridactylus in an embryo. 



Prom Hyrtl's observation, and from the distinct and complete fusion of the crura in 

 the stapes on one side only in the specimen already referred to, we .see in the Ai a 

 singular wavering between the higher and lower type of this ossicle in the Vertebrata 

 that possess it. The intercrural aperture in none of the College specimens is as large as 

 it is in the columella of 'the Golden Eagle (Aquila clirysaetos), so that the presence of that 

 space must not be considered unreservedly as a tendency to higher type. 



The ossicula of the Unau ( Clwlwpus didactylus) differ from tHe above in some respects 

 (see PI. LXIY. fig. 2). The head of the malleus is even more developed, and is some- 

 Avhat produced forwards, as in the Hystricidse. The articular surface is similar ; the neck 

 is rather longer, but more curved, assuming the sigmoid form of the ridge on the human 

 malleus. The lamina is similarly represented by a narrow tract of rather thick bone. 

 Indeed the malleus of both the Sloths is distinctly intermediate between the non-lami- 



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