486 THE VERTEBRATE SKELETON. 



as a whole to the hyomandibular of fishes and to the columellar 

 chain of Sauropsids and Amphibians. The malleus is homo- 

 logous to the extracolumella of Crocodiles and the stapes 

 to the columella. The malleus when typically developed 

 consists of a rounded head (fig. 100, 1) which bears a surface 

 articulating with the incus, and a short neck continued into a 

 process, the manubrium (fig. 100, 5), which comes into relation 

 with the tympanic membrane. From the junction of the neck 

 and manubrium two processes are given off, a processus longus 

 or gracilis (fig. 100, 4), which in the embryo is continuous 

 with Meek el's cartilage, and a processus brevis (fig. 100, 6). 

 The incus generally consists of a more or less anvil-shaped 

 portion which articulates with the malleus, and of a process 

 which is connected with the stapes by the small lenticular. 

 The stapes is generally stirrup shaped, consisting of a basal 

 portion from which arise two crura separated by a space the 

 canal through which a branch of the pharyngeal artery runs 

 The lenticular is frequently cartilaginous and sometimes is 

 not developed at all. 



The above is the arrangement of the auditory ossicles met 

 with in the higher Mammalia, but in the lower Mammalia 

 the characters approach more nearly to those met with in 

 Sauropsids. 



In MONOTREMES the ossicles, though distinctly mammalian 

 in character, show a very low type of development. The incus 

 is articulated, or often fused, with an outgrowth from the 

 head of the malleus. The stapes is very much like a reptilian 

 columella, having a single crus with no perforation. 



In MARSUPIALS the ossicles are of a low type, but not so 

 low as the rest of the skeleton might have led one to expect, 

 and all or almost all the points showing a low grade of de- 

 velopment may be paralleled among the Monodelphia. The 

 lowest Marsupials as regards the ossicles are the Peramelidae, 

 whose ossicles are of a frail papery consistence. The Didel- 

 phyidae on the other hand have the most highly developed 



