410 MR. A. H. G. DORAN ON THE MORPHOLOGY 



trace of a neck and a characteristic manubrium, very thin, very broad at the base, and 

 still broader at the middle, where an angular projection from the inner border constitutes 

 the processus muscularis. There is a distinct angle corresponding to the processus brevis ; 

 and the outer aspect of the manubrium, though very narrow, is distinctly bordered off 

 from the sides, as in the Carnivora ; the extremity is distinctly spatulate. The malleus of 

 both Myoxus glis and M. muscarclinus much resembles the same among certain Muridse, 

 where the lamina, conspicuous in Mus, is nearly suppressed. 



The incus in the Dormice has a longer and more slender processus longus than in 

 other families of Bodents ; the posterior crus is short and very thick. 



The stapes has very slender and straight crura, neither of which is inserted into the 

 extremity of the base, which is quite plane. No bony canal exists between the crura in 

 the two species above mentioned. 



In the Beavers, Castorid^; (PL LX. fig. 12), the head of the malleus is still large in 

 proportion to the size of the whole ossicle, as in all the above families ; it is also rather 

 flattened laterally : the articular surface is very wide vertically, as in the Marmots ; but 

 the head does not rise high above it. There is a narrow lamina between the head and 

 the processus gracilis, and a distinct, much constricted neck. The manubrium is very 

 broad laterally; at the outer aspect of the base is a very prominent angle (pb); it is 

 recurved and spatulate at the tip; but the processus muscularis (pm) is placed close 

 to its root, so that there is no angular projection on the inner border. Such is the case 

 in many Pats and Hares ; but in the form of the head of the malleus the Beaver more 

 resembles the Squirrels. 



The processus brevis of the incus is very short and pointed, it diverges distinctly, as in 

 most of the Muridse. The body is square and well developed ; the processus longus is 

 rather long, and supports a well-developed Sylvian apophysis. The stapes of the Beavers 

 is peculiar, it is large, with very slender, widely divergent, and rather short crura, both 

 inserted some distance from the corresponding extremity of the base, which is very large, 

 being long horizontally and wide vertically. There is no bony intercrural canal. 



In the Mtjrid^;, Pats and Mice, the malleus retains the singular form seen, with modi- 

 fications, in Arctomys, Castor, &c, in its less typical genera ; but in the genus Mus and 

 allied genera the broad lamina seen in Sorex, the Carnivora., and the Ungulata reappears. 

 Even in more aberrant genera the lamina is always developed, though to a less degree. 



In Mus decumanus (PL LX. fig. 14) the head is little developed and deeply excavated 

 by the articular surface ; the neck is long, thin, and much curved, and, together with 

 the head, is united to the processus gracilis by a broad area of lamellar bone (I) ; this 

 lamina is bordered above by a linear prolongation of compact bone from the front 

 of the head (phm), the processus gracilis bordering it anteriorly and below. Prom 

 the outer aspect of the neck projects an orbicular apophysis (oa), a prominent 

 spherical knob of bone, as seen in the Shrews. It displaces the manubrium considerably 

 outwards ; and, as in Sorex, it is probably an extreme development of the boss sent 

 outwards from the shoulder of the incurved capitular portion of the first postoral 

 bar *. The tensor tympani is not inserted into it, but into a minute tubercle close to 



* " On the Structure and Development of the Skull of the Pig," by Prof. W. K, Parker, F.R.S., Phil. Trans. 

 1874, p. 301. 



