OF THE MAMMALIAN OSSICULA AUDITUS. 475 



the head is very ill developed ; the column does not exhibit the least trace of any division 

 into crura ; the foot-plate is almost circular, and in diameter almost equals the length of 

 the column. A tendency to this form of the base has already been described in Bradypus ; 

 and the gradations from the completely bicrurate stapes of Busy pus, through the form 

 seen in Tolypeutes, to that in Munis are most instructive to observe. 



In Manis tricuspis, M. longicauda, and If. pentadactyla, the malleus is identical in form 

 with that of M. dalmaiiii ; but in the incus (PL LXIV. fig. 9) the portion of the body over 

 the processus brevis is extremely high, so that the whole ossicle assumes a form seen in 

 Mycetes and Pithecia ; the processus brevis is thinner and slightly more divergent than 

 in If. dalmanii, though both processes are as short. The stapes has a shorter column, 

 without any trace of division into crura, and it is absolutely shorter than the diameter 

 of the foot-plate, which is not so circular as in the species first described. 



In the Great Ant-Eater (Myrmecophaga, PL LXIV. fig. 11) the malleus has a large head, 

 very much flattened laterally and produced forwards ; its upper portion, very wide antero- 

 posteriorly on account of the lateral flattening, does not project at all above the level of 

 the articular surface, which is rather deeply cut, and much wider vertically than from 

 without inwards. The lower facet is the most concave, and is divided from the upper 

 by a very faint groove. The neck is long and almost straight ; there is a very narrow 

 lamina ; the manubrium forms an angle of about 150° with the neck. The handle is 

 long and slender; there is no processus brevis, but a very blunt angle in its place;, the 

 extremity is very slightly spatulate and scarcely recurved. The sides are flattened, and 

 not broad near the base ; there is no trace of any processus muscularis.. This malleus of 

 the Great Ant-eater bears a certain resemblance to that of Briodon^ from which it may 

 at once be distinguisbed by the form of the head.. 



The body of the incus of Myrmecophaga (fig. 11) is almost square, although rather 

 broader than deep. The processus brevis is moderately long, divergent, and blunt- 

 pointed ; the processus longus is well developed without being very stout, and bears a 

 distinct elliptical Sylvian apophysis on a broad pedicle. The whole bone is large in 

 proportion to the malleus. 



The stapes (fig. 11) has an ill-developed head and long very slender crura, with a 

 wide aperture between them. The base is thin, but very broad vertically as well as 

 horizontally. Thus this bone in the Great Ant-eater approaches the type of Dasypus 

 rather than Brady pus, Tolypeutes, or Manis. The vertical breadth of the foot-plate 

 (that is to say, the tendency to roundness in that portion of the ossicle) is the only 

 approach towards the Sauropsidan columella, the crura tending towards much higher 

 types. 



In Tarnandua tetradactyla (PL LXIY. fig. 12) the head of the malleus, though flat- 

 tened laterally, projects much more above the upper margin of the articular surface 

 than in Myrmecophaga, so as to be very convex above. The neck is a little more 

 curved, and the lamina wider ; the manubrium is very similar in form. The incus has 

 an even squarer body ; its processus longus bears a similar Sylvian apophysis. 



In Cyclothurus didactylus (PL LXIV. fig. 13) the head of the malleus is as prominent 

 above as in Tarnandua ; the neck is shorter, and the manubrium is much broader at the 



