OF THE MAMMALIAN OSSICULA AUDITUS. 467 



The stapes of the Dugong (fig. 30) measures about half an inch in length, and is of 

 very solid consistence ; as the crura diverge but little, the base is narrow ; the anterior 

 extremity of the latter and of the fenestra ovalis is placed much lower than the posterior, 

 although, after examining five skulls of Halicore in the College Museum, I do not find 

 the long axis of the base of the stapes or of the fenestra vertical, as has been asserted ; the 

 same remark applies to Manatus. The head is well developed, and the articular surface 

 is concave ; the site of insertion of the stapedius is generally indicated by a slight depres- 

 sion. The anterior crus is half the thickness of the posterior, and both are rather 

 crooked ; the aperture is circular, generally just large enough to admit a small sewing- 

 needle, but in some specimens it is of the calibre of a No. 1 English catheter. The base 

 is convex, most so posteriorly, and is narrowest towards the front ; it never appears to be 

 ankylosed to the fenestra ovalis, or even tightly fitted into it, as in the Cetacea. The 

 broad well-formed head and comparatively thin anterior crus somewhat remind the 

 observer of the stapes of Bos ; but in the other ossicula of Halicore and the other Sirenia 

 few, if any, distinct analogies to the same bones in any particular order of Mammalia can 

 be made out. 



Since the extinction of the Eliytina the American Manatee {M. americcmus) has borne 

 the largest and heaviest malleus known in any animal, as Hyrtl has already stated. 

 Indeed, to judge from Claudius's account of that ossicle in Bhytina, of which the head, 

 he remarks, is of about the size of an ordinary hazel-nut, it hardly can have exceeded, 

 as a rule, the malleus of a large Manatee, though I have never seen the processus 

 gracilis of the latter " at least 1J"' in diameter," which is the case in the specimen from 

 the extinct Sirenian he describes. The specimen in the collection of ossicula in the 

 College Museum (PL LXIII. figs. 21, 22), from a skull of M. americanm, in the Com- 

 parative-Anatomy series, measures over an inch in length, and weighs over sixty-one 

 grains ; the head is quite as large as a middle-sized hazel-nut. This bone, as well as the 

 other ossicula, lies, in the recent skull, in the same position as has already been described 

 in Halicore ; but it offers many points of difference in its structure from the malleus of 

 the latter animal. 



The head is much longer antero-posteriorly than vertically. All the facets are larger ; 

 and the upper, though reniform, has its concavity backwards, and its surface is distinctly 

 concave above. In the College specimen there is a prominent peg-like process (PI. LXIII. 

 fig. 21) projecting immediately below the inferior border of this facet, bearing supe-. 

 riorly another small facet; not a trace of this can be seen in any specimen from 

 M. senegalensis in the same series. The lower facets in Manatus americanus are of 

 much the same form as in the Dugong ; but at the top of the prominent angle their 

 borders do not blend, but are nearly a line apart. The outer aspect of the malleus is 

 very wide and quite convex ; the groove is very shallow anteriorly, and quite effaced 

 behind, in which direction, however, is a very sharp and prominent ridge analogous to a 

 similar projection in Halicore, the homologies of which have already been discussed. 

 The processus muscularis is a stout incurved process, continued backwards as a con- 

 spicuous elevation towards the former ridge, precisely as in the Dugong. 



