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



The manubrium differs in form from that of Halicore. The upper border is very sharp 

 and narrow; it forms a semicircle, and projects against the membrana tympani in the 

 live subject ; the outer border is short, and its margins, united above, diverge but little 

 towards the extremity ; its surface is concave vertically, and slightly convex horizontally. 

 The inner border is much blunter than the outer in Manatus. 



The processus gracilis is not relatively so well developed as in Halicore, and projects, 

 as a fragile splinter of bone, from the massive head; it is firmly ankylosed to the 

 tympanic ring ; and, from its very nature, the whole malleus is easily broken off from 

 the ring. 



The incus (figs. 23, 24) is placed in the tympanic cavity precisely as in the Dugong, 

 the processus longus lying above the body, and the other crus running downwards from 

 it. The body is even shallower and wider between the crura than in Halicore ; its inner 

 surface is less extensive and less convex, and much overhung by the upper facet of the 

 articular area, which facet is very wide, reniform, and faintly concave ; the two lower 

 facets look towards each other, and are separated by a deep groove, into which the cor- 

 responding prominence between the opposing surfaces on the malleus are fitted during 

 lifetime. There is not nearly so marked a prominence behind the upper and more 

 posterior of these facets in the Manatee, like that which projects so conspicuously on the 

 posterior side of the Dugong's incus. The processus longus is rather long, slightly 

 curved, and flattened extero-internally ; it widens out towards the point, where it is 

 ankylosed to the petrous bone. The processus brevis is very short, stout, and curved ; 

 the articular surface for the stapes is very wide, almost circular, and nearly plane ; a 

 faint trace of a rim can be seen above it, separating it from the crus, as the homologue 

 of the os orbiculare. 



No stapes of this Manatee exists in the College collection; but that of M. senegalensis, 

 of which there are several examples, will be shortly described. The malleus from an 

 adult of the latter species is rather smaller than in the American kind ; the upper facet 

 is not so wide, and there is no peg-like projection internal to it. The processus muscu- 

 laris is extremely stout and prominent ; but the manubrium is not so broad as in 

 M. americanus. 



The incus of Manatus senegalensis very closely resembles that ossicle in the American 

 Manatee ; the processus longus is of similar form and length ; the articular surface is 

 slightly different, in accordance with the characters of that surface on the malleus of the 

 African species ; hence the upper facet is smaller, and does not overhang the inner side of 

 the body as much, and the two lower facets gape wider than in the American species. 

 In none of the existing Sirenia does the incus reach the size of that of the Seal 

 {Macrorhinus proboscideus), where it is larger than in any other animal. 



But as is the case with the malleus, so is the stapes of Manatus (fig. 25) largest among 

 mammalian stapedes. It is over an inch in length, and weighs about one drachm ; it is 

 of the dense consistence of the other ossicula in this Order. In shape it differs from the 

 stapes of Halicore, being over twice as long as it is wide. The head is well developed, 

 and bears an almost circular articular surface. Both crura are long and hardly 



