416 - MR. A. H. G. DORAN. ON THE MORPHOLOGY 



section of the Rodentia, a narrow rudimentary lamina (I). The incus of Octodon has 

 a well-formed body and slender crura. The crura of the stapes are very thin ; its base is 

 wide; there is a dense bony intercrural canal in the recent skull*. 



In the Chinchillid^e, the malleus of Chinchilla lanigera (PI. LX. fig. 29) has a head 

 extremely produced, so as to look like a slightly flexed index finger, a very charac- 

 teristic feature. The manubrium is very broad, resembling that of Aulacodus rather 

 than that of Hystrix, the processus muscularis being on the inner border close to its 

 root and not further down. This ossicle is very completely fused by its articular surface 

 to the incus, of which the body is well developed ; and both crura are stout and short. 

 The crura of the stapes are very slender ; and there is a distinct bony canal between them 

 in the recent skeleton. 



Among the Dastproctid^e, the Spotted Cavy {Ccelogenys paca) has the malleus and 

 incus completely fused. I have found, in a mutilated specimen prepared for the 

 College collection, that the line of demarcation between the ossicula was best-marked 

 above and internally ; Hyrtl describes it as best-marked below and only partially indi- 

 cated above. The head of the malleus is not so prominent forwards as in Hystrix. 

 There is no canal between the crura of the stapes in the recent skull. 



In the typical genus Dasyprocta (PL LX. fig. 30) the fused malleus and incus (anky- 

 losed even in very young skulls) closely resemble the same in Hystrix. The head of the 

 malleus, however, is less produced, and the two otosteals are more flattened than in the 

 Porcupine. The stapes is small in proportion to the other ossicles ; the crura are short, 

 straight, and not widely divergent, so that the aperture is small. The base is very broad, 

 projecting far beyond the insertions of each cms. In one out of three skulls of D. aguti 

 I have found a bony intercrural canal ; none existed in a cranium of B. cmtillensis. 



In the Patagonian Cavy {Dolichotis patachonica, PI. LX. fig. 31) the malleus and 

 incus are completely ankylosed. The head of the malleus is stouter and straighter, and 

 at the same time more produced than in Dasyprocta. The boundaries of the articular 

 surface are clearly marked on the outer, but very faint on the inner aspect of the fused 

 bones. The manubrium of this representative of the Caviidje is trapezoidal as in Hystrix. 

 The angle at the outer aspect of the base is very obtuse, but still it represents the 

 processus brevis (pb); the well-developed and tubercular muscular process (pm) is 

 seated on a sharp angle on the inner border, which is very thick above and quite sharp 

 below that process. The outer border broadens regularly from the processus brevis to 

 the tip, which is slightly everted and spatulate, its edges spreading out widely from the 

 thin compressed sides of the manubrium. The body of the incus is both broad and deep ; 

 and its crura are of almost equal thickness and length. The stapes is proportionally 

 large, the crura very straight, rather long, and divergent; they are deeply grooved 

 within ; and in the recent skull a bony canal runs between them. The base is broad, 

 extending considerably beyond the insertions of the crura ; it is convex towards the 

 vestibule ; the convexity is separated from the periphery by a deep channel, so as to 

 form a large umbo. 



* In the figure of the malleus of Octodon the letters pm have been placed where pg should have been written; 

 and I is marked too close to the head of the malleus. 



