OF THE MAMMALIAN 0SS1CULA AUDITUS. 489 



of the incus. This scale hides the concave excavation of the head, which, on the other 

 hand, is plainly visible from the inner aspect of the united bones (fig. 36, hm). 



From, the inner aspect, too, it may be seen that the head sends forward a long, pro- 

 minent, and very straight process to join the processus gracilis *. The analogue of this is 

 already familiar to us in most laminated and in other mallei. The neck is rather long, 

 and not strongly curved ; its free border appears from in front of the incus when the 

 bones are viewed from within, as just described; in the opposite direction it blends 

 insensibly with that part of the hone representing the lamina in higher mammals. 

 From the outer aspect the neck is more clearly defined. Close to the root of the 

 manubrium a small tubercle rises from the inner side of the neck, to which the tensor 

 tympani muscle will be found attached in imperfectly macerated specimens. Hence 

 this tubercle is the processus muscularis. 



The manubrium of Echidna is of a type very frequent among the lower mammalia. 

 It is of moderate length, compressed laterally, but not so thin and broad at the root as 

 in some Rodents. The outer angle of its hase (the site of the processus brevis, wdiich is 

 absent) is very blunt, broad, and well rounded ; the outer aspect is very narrow, though 

 sharply bordered from the sides, and ending in a narrow but distinct spatulate dilata- 

 tion. The entire process, in slenderness, bluntness of its outer angle, &c, recalls the 

 same in Rhinoceros, Priodon, and the Marsupials. It cannot be said to bear any close 

 resemblance to any process of the os quadratum or any neighbouring bone in the 

 Sauropsida. 



The processus gracilis is readily distinguished, as it starts off from the malleus, forms 

 a deep curve, and joins the process from the head, to form the long straight splinter of 

 hone which articulates with the tympanic. This condition is well known ; but anatomists 

 hitherto have described the united part as if it were solely and entirely the processus 

 gracilis, ignoring the process from the head, which I have frequently described in 

 animals where it is well marked, notably in Fiber. Whilst this latter process, as above 

 stated, is traced along the united portion from the inner aspect, the continuation of the 

 processus folianus proper must be looked for in viewing the malleus from without ; then 

 it can be seen to terminate in a broad lamellar expansion, on which the process from the 

 head loses itself posteriorly. 



Between the neck and the free curved portion of the processus gracilis (and hence best 

 seen from the outer aspect of the ossicle) the analogue of the lamina so frequent in the 

 higher mammals may be distinguished : it is neither broad nor thin, and runs on towards 

 the united processes, with which it hlends insensibly. This, it may be remembered, is the 

 same as in the Marsupials. 



The body of the incus has already been referred to, and the free inner aspect described, 

 as well as the manner in which it is wholly covered externally by the malleus t. The 

 processus brevis is short and flattened extero-internally ; it projects from the outer 



* See PI. LXrV. fig. 36. The process from the head (jphn) is here distinguished from the processus 

 gracilis (pg). 



f Excepting its anterior edge, which is just left uncovered. 



