OF THE MAMMALIAN OSSICULA AUDITUS. 477 



Bradypus and Dasypus than do Myrmecophaga and Tamandua ; and its stapes, in one 

 young specimen at least, approaches much more to the Sloth type. 



V. In Orycteropus the malleus, quite unlike that of any Ant-eater, Sloth, or Manis, 

 approaches to a certain extent that of Priodon, from which it has, on the other hand, 

 very distinctive features. The stapes is of as high a type as in Dasypus. 



VI. That many instances are observed in this Order of the ossicula of the adults of 

 some genera resembling those of the young of others ; especially does the malleus of 

 Priodon or Tolypeutes, however old and well developed the specimen it be removed 

 from, remind the observer of the malleus of a new-born or foetal Dasypus or Tatusia. 

 The highest degree of development is not necessarily seen in the largest species, as is 

 at once evident on comparing the malleus of Priodon gigas with that of Dasypus 

 minutus. 



The Ossicula op the Didelphia. 



The study of the ear-bones of the Marsupials is of necessity peculiarly interesting, as 

 it is on them that we must depend to a great extent for elucidation concerning the 

 obscure homologies of the proximal elements of the mandibular and hyoidean arches in 

 the Mammalia and Sauropsida. Considering how undoubtedly inferior these animals 

 are to placental mammals, it must be clearly understood that their auditory ossicula 

 are, on the whole, of proportionally high type ; for though these little bones in adults of 

 this subclass resemble, as a rule, then representatives in the foetus of many higher 

 animals, still hardly one persistent embryonic feature observable in any Order of the 

 Didelphia is not also to be found in many highly organized Monodelphia. Indeed, in 

 large Orders, as the Ruminants, or smaller ones with families very specialized and 

 different in important features from each other, as the Edentata, certain genera, as 

 already shown, retain in a most remarkable and instructive manner characters which 

 disappear during advance to maturity in genera closely allied to them, where the species 

 may be either larger or of distinctly higher conformation. 



Those families will first be described where the stapes has divergent crura, as this 

 character is manifestly a high one. But it must here be remarked that the divergence 

 is slight, and only commences a little above the base-plate ; never has any Marsupial 

 been found to possess a stapes with a wide aperture extending freely in the direction of 

 the head as in Man, most Insectivora, and many Rodents ; nor can examples be found 

 where, as in the Cetacea, the smallness of the aperture is due rather to thickness and 

 over-development than to non-divergence of the crura. On the other hand, the inter- 

 mediate condition seen in this section of the Marsupials exists already in many mono- 

 delphous mammals, as in Priodon, Tolypeutes, and to a certain extent in Rhinoceros and 

 the Marmosets, as the author has already indicated. The families where this type of 

 stapes is found are the Macropodidse and the Didelphyidse. 



Secondly the ossicula of those families will be described where the stapes is absolutely 

 columelliform, without any aperture or any indication of divergence of crura. Even this 

 still lower type is represented in higher mammals, namely in Manis. The stapedes of 



SECOND SERIES. — ZOOLOGY, VOL, I. 3 S 



