ON THE MAMMALIAN OSSICULA AUDITUS. 469 



divergent; both are crooked; but the anterior is the thinner and straighter; the 

 posterior is very broad inferiorly, often bulging prominently just above the base. The 

 aperture appears as an oblique slit, a quarter of an inch long from above ; but on the 

 lower aspect it is more circular : in a large specimen it is of the calibre of a darning- 

 needle. The sides of the crura bounding it are slightly concave, but not absolutely 

 grooved. The base is rather narrower than are the crura a little above it, but it is 

 wider than the head ; it is convex towards the vestibule ; and its margin is generally 

 rather concave above, in accordance with the form of the fenestra ovalis in this animal. 



According to Claudius *, the malleus and incus of Rliytina closely resemble those of 

 Manatus. In the work already referred to, these ossicles are figured and described ; and 

 that anatomist does not name any point in which the extinct differs from the above- 

 named existing Sirenian as far as these bones are concerned. The inner border of the 

 manubrium, he says, is shorter, narrower, and sharper than the outer (that is, the upper 

 and outer surface of our description) ; but he does not state whether the latter is broad, 

 with sharply defined margins, as in the Dugong, or sharp and narrow, as in the Manatee. 

 The incus is placed, according to his account, precisely as in the existing species ; and 

 the short crus is ankylosed to the petrous bone ; the processus longus is described as 

 long and strong, though in the plate it does not appear to be longer than in Manatus. 



Dr. Krauss t has had the good fortune of procuring and examining all three ossicula 

 of Halitherium schinzi, which he figures both separate and articulated. Prom the 

 letterpress and the plates it appears that the malleus is intermediate in form between 

 the Manatee's and Dugong's. The body is figured as short antero-posteriorly, as in Hali 

 core ; but the manubrium is represented more of the Manatus-tyj)e ; the articular surface 

 though not distinctly described or figured, must, judging from the incus, be as in the 

 latter animal ; the processus gracilis is rather long, thin, and brittle. As for the incus, 

 it is both described and figured as much more like that of Manatus than Halicore ; the 

 articular surface is represented as large as in the Manatee, and similar in its facets ; more- 

 over there is no prominent tubercle on the outer aspect of the body, backing up one of 

 the facets, as I have observed and described in Halicore. The stapes is described as 

 very similar to that of Halicore, " whilst that of the Manatus is much larger, thicker, 

 and rounder." The illustration confirms this statement, but represents the crura as 

 rather longer than in the Dugong ; the anterior crus is much the thinner ; the head is 

 broad ; and the whole bone decidedly, as far as one can judge from a plate, approaches 

 the quadrilateral type of the larger Ruminants. 



In conclusion, it may be said of the ossicula of the Sirenia : — 



I. That their dense structure and clumsy form, consistent with the characters of the 

 whole skeleton, distinguish them at a glance from their homologues in all other Orders. 



II. That their modifications lie much more in their peculiar general conformation than 



* Mem. Acad. Imp. des Sciences de St. Petersbourg, 1867. 

 t As already quoted in the Jahrb. 1862. 



Sr 



