OSSEOUS SYSTEM OF MAMMALIA. 313 



in the Mammal mainly to the support of the ear-drmn, the accessory 

 function with which it is charged wholly or in part in all air- 

 breathing Vertebrates. 



Remove 27 and 28 from the cranium of the Bird and Monotreme, 

 as in figs. 196 and 197, and the homology of the remaining cranial 

 bones, especially of 2, 3, 8, 6, is unmistakable. The mastoid, 8, 

 in both Bird and Monotreme, is developed from cartilage ; arti- 

 culates posteriorly with 2, 3, superiorly with 7, anteriorly with 6 ; 

 coossifies internally with the petrosal, and gives attachment 

 inferiorly to the bone, 28, which supports wholly or in part the 

 ^ membrana tympani.' The squamosal, 27, is a backward prolon- 

 gation of the bar, 2fi, attaching the upper jaw to the tympanic; it 

 is developed in the embryonal scaffolding external to the proper 

 cranial cartilage ; it articulates posteriorly Avith the tympanic, 

 28. It forms no part of the outer wall of the cranium in Birds, 

 and is equally excluded from that cavity in Cetacea, fig. 198, in 

 most Ruminants, fig. 140, 

 and in many Rodents : the 

 supplementary function 

 of completing such cra- 

 nial wall is peculiarly 

 mammalian, and does not ^^JLc '-i^ '^ 

 supersede the share taken ^^%X^J^-1 

 in such lateral wall by ^^ ^' 



* Section of cranium, Porpoise. 



8 and 6, ni all Verte- 



brates. Moreover, 27 constitutes the hinder and major part of 

 the zygomatic arch in both Birds and Mammals, as in most Rep- 

 tiles ; with such homologically unimportant modifications of shape 

 as are exemplified in the Turtle and Crocodile (Vol. I. figs. 91 

 and 95, 27), and in the figures 26, 140, 196, 197, 27, of the present 

 Volume. 



The bony pedicle wdiich suspends the mandible to tlie side- 

 processes of the cranium, is that which is marked 28 in the Fish 

 (Vol. I. figs. 81, 84), the Serpent(figs. 96, 97), the Tortoise (figs. 91 

 and 92), and the Crocodile (figs. 93, 95). As those side-processes 

 are homotypes of the transverse processes (par- di-apophyses) of the 

 trunk-vertebrge, so 28 bears the same serial relation to the ' pleura- 

 pophyses,' the mandibular rami completing ' hajmapophysially ' the 

 inferior or haemal arch of the cranial segment. This vertebral 

 character is shown in tlie developement of the Vertebrate skull : 

 the simple rib-like cartilage formed in the second (counting back- 

 ward) of the embryonal, ' visceral,' or haemal arches, manifests 

 always its upper or ' pleurapophysial ' and its lower or ' hajmapo- 



