OF THE MAMMALIAN OSSICULA AUDITUS. 411 



the root of the neck, and just below it. The relations of the processus muscularis to 

 the orbicular apophysis will be considered when the ossicula of the Shrews are described, 

 as certain interesting homologues are seen among the Iusectivora, throwing much light 

 upon the subject. 



The manubrium is much flattened laterally, very broad at the base ; and, as in Castor, 

 the processus muscularis is at its root, not halfway down its inner margin. The pro- 

 cessus brevis is an angular projection concealed by the orbicular apophysis, which lies 

 above and internal to it. 



The incus has an ill-developed, slightly divergent, short crus. The processus longus is 

 stout, and projects far forwards. 



The stapes, as in other Muridae, has slender, long crura, not very divergent towards 

 the base, which is thin and narrow ; there is no intercrural canal. 



Mus ratttis closely resembles the above in its ossicles ; in M. musculus the orbi- 

 cular apophysis is very well developed, and was first described in that animal by 

 Hagenbacb. " 



In the Golden-bellied Beaver-Rat (Hydromys chry so g aster) the malleus, being larger, 

 shows the murine characters very well (PI. LX. fig. 16) ; the spherical knob of bone is 

 proportionally less developed tban in the smaller mice. Below this orbicular apophysis 

 a tubercular projection can be detected; this is the real processus muscularis; I have 

 seen the tendon of the tensor tympani still attached to it. The processus brevis of 

 the incus is very short. 



In Hapalotis mitchelii (PL LX. fig. 15) the orbicular apophysis is ill developed, and 

 the tubercular processus muscularis, close to the root of the manubrium, is plainly 

 visible with the aid of a pocket-lens. The manubrium itself is rather shorter than in 

 Mus. The anterior crus of the incus is far stouter tban the posterior. The stapes is of 

 the same form as in the past genera of the Muridse ; and, as in them, there is no bony 

 canal between the crura. 



In the Water Voles (Arvicola, PI. LX. fig. 17) the lamina of the malleus is less deve- 

 loped, the orbicular apophysis is quite absent, and the manubrium is rather short and 

 very broad at the base. The processus muscularis is tubercular, and is placed on the 

 inner side of the neck, close to the manubrium, as in Mas. In A. amphibias I have 

 found a bony canal between the crura of the stapes, but not in A. arvalis. 



In the Musquash (Fiber zibetliicus, PI. LX. fig. 18) the head and lamina of the malleus 

 resemble those of Arvicola ; but from the front of the head a very stout process of bone 

 {\h.p h m) runs to join the upper border of the lamina : as the true processus gracilis is 

 very slender, the former might, on hurried observation, be mistaken for it. This process 

 from the head exists in the mallei of most mammals, but rarely to so highly developed 

 an extent as in Fiber. Its homologue will be seen to form a conspicuous feature in the 

 Cetacean malleus. The manubrium is extremely broad and flattened, even for a Rodent ; 

 the processus muscularis is at the root of the manubrium, as in Mus. The incus has an 

 ill-developed processus brevis, and a long, divergent processus longus. 



The stapes has the same form as in Mus and the other Muridse, the crura being long, 



