OF THE MAMMALIAN OSSICULA AUDITUS. 491 



the stapes of these inferior mammals represent the same or only a part of the elements 

 making up the column of a bird's columella ? 



In all these mammals we have an incus with a processus longus and Sylvian apo- 

 physis articulating with the stapes, which is columelliform ; but the column is clearly 

 identical with the two crura of the higher Mammalia, as the transitional condition 

 seen in Macropus &c. clearly proves. 



In birds the column of their stapes or " columella " is certainly the same, whether 

 short (as in the Finches, PI. LXIV. fig, 44) or very long (as in the Eagle, Albatross, &c, 

 figs. 46, 47) ; for the same " extrastapedial " and " suprastapedial " elements are found 

 attached to its outer extremity in the same manner in both varieties. But at least part 

 of the column is recognized by the latest investigators into embryology * as the " medio- 

 stapedial" element which represents the long crus of the incus in mammals,., and 

 begins as the proximal incurved extremity of the hyoid arch in both the Mammalia 

 and the Sauropsida. 



Now, this long crus of the incus always existing in mammals, it follows, if the most 

 advanced embryologists have not erred, that the column of the Marsupial and Mono- 

 trematan stapes (as well as that of Manis) represents only a part of the columella in 

 Aves. It is evidently formed entirely from the mass of cells which grow from the 

 periotic capsule to meet the proximal end of the hyoidean arch (Parker, I. c.) ; for since 

 that end becomes the processus longus of the incus and its Sylvian apophysis in these 

 mammals (identical with the mediostapedial element in birds), no segment of that 

 arch can take any share in the formation of their stapes, as it is seen to. do in the 

 Sauropsida. 



In examining the ear-bones of Ornithorhynchus paradoxus, the first thing that strikes 

 the observer is the relatively small size of the malleus and incus compared with those bones 

 in Echidna, and. their imperfectly ossified semimembranous texture, in which they exceed 

 those of any other mammal. The stapes in the Duck-bill is nearly as large as in the 

 Porcupine Ant-eater (PL LXIV. fig. 37). 



If the united malleus and incus of Ornithorhynchus be more closely investigated, it 

 will be found that in ossicles from young skulls they may be readily disarticulated; 

 but in specimens from the adult this is not invariably the case. Professor Peters t 

 asserts that these bones remain free throughout life in this animal ; but in a set which 

 I prepared for the College collection, from a full-grown skull, the malleus and incus 

 remained ankylosed on the right side. 



The manner of union between these two ossicula is precisely the same as in Echidna. 

 The incus is seen from the inner aspect (PI. LXIV. fig. 38), but almost completely 

 concealed in viewing the malleus from without (fig. 37)+. 



The portion of the malleus representing the head in higher mammals (hm) is ex- 



* Consult summary at the end of Professor Parker's memoir " On the Structure and Development of the Skull of 

 the Pig," Phil. Trans. 1873. 



t Loc. cit. 



j In PL LXIV. figs. 37 & 38, the same abbreviations are used as in the figures of the corresponding ossicles 

 in Echidna. 



