OF THE MAMMALIAN OSS1CULA AUDITUS. 401 



of the body of the incus is not much higher than the anterior, the latter being well 

 developed ; the anterior or long crus stretches well forwards, and supports, as in the 

 other Eared Seals, a Sylvian apophysis in the form of a perfectly circular sessile disk. 

 The crura of the stapes are quite fused, and the base is very thick ; the bone strongly 

 resembles the same in the Dolphins. In Otaria falklandica (PI. L1X. figs. 18, 19) 

 the depression in front of the head of the malleus is deep and well defined, the neck is 

 very short and constricted, the manubrium shorter than in Steller's Sea-lion, though, 

 from the ill-development of the neck, it looks as long. The body of the incus is not 

 quite so broad as in the last two species, the posterior part is much higher than the 

 front, the processus longus does not reach so far forwards as in O. stelleri and jubata. 

 The two crura of the stapes are very thick and straight, with a distinct space between 

 them, and a base like that in O. stelleri. It is more regular in form than in any true 

 Seal ; but a stapes of Phoca groenlandica in the College collection makes a near approach 

 to it, excepting that the ossicle in the Greenland Seal is much larger. 



In the "Walrus {Trichechm) (PL LIX. figs. 21, 22) the malleus has a large broad head, 

 of a rhomboidal form, its anterior and posterior aspects being very much flattened, and 

 very wide in the middle, tapering at their upper and lower extremities. The former 

 surface bears a deep concavity, as in Otaria and the Phocidse ; but, unlike either, there is 

 a smooth rounded surface, of small extent, above the level of the articulation, repre- 

 senting the globular part of the head seen in other mammals. Posteriorly lies the 

 articular surface, as broad horizontally and as narrow vertically as in the Seal, but 

 concave in both directions ; the facets are only slightly saddle-shaped, and divided by a 

 very distinct but shallow groove. The neck is short, not broad near the head, and 

 extremely constricted close to the manubrium ; it is also flattened antero-posteriorly. 

 It bears no vestige of a muscular process ; and the root of the processus gracilis is very 

 blunt. The manubrium, which forms little more than a right angle with the neck, 

 is extremely short ; it is proportionally broad at the base and at its outer aspect; there 

 is a distinct everted processus brevis at its upper extremity. The whole bone is not so 

 large in proportion to the whole skeleton as in the true Seals, but is larger than in any 

 Otaria. It is plainly a mere modification of the Seal-type of malleus, the articular 

 surface being the most distinct feature ; the shortness of the malleus is only a slight 

 exaggeration of the condition seen in Stenorhynchm, where, on the other hand, the neck 

 is shorter than in Trichechm. It far less resembles the Otarian malleus. 



The incus is a large bone, with the posterior part of the body well developed and 

 bulging externally ; the processus brevis is long, thin, and rather divergent. The pro- 

 cessus longus is stout, stretches far forwards, and is broadest in the middle, without a 

 trace of a groove on its broad inner aspect. It forms a gentle sigmoid curve, and bears 

 on its extremity a thick, sessile, discoidal Sylvian apophysis. This bone, too, is very 

 seal-like in the thinness of the processus brevis. The long, far-reaching processus 

 longus allies it to Otaria ; but I have examined one incus of Trichechm where that crus 

 was rather short ; and moreover divergence and length is seen in the same process in 

 the Cystophorine Seals, but masked by the huge over-developed body of the incus. 



The stapes is a high narrow bone, with a small head, thick crura, completely fused, 



