OSSEOUS SYSTEM OF MAMMALIA. 303 



autogenous element by a distinct centre of ossification ; in most 

 Mammals it speedily coalesces with the petrosal, but not in the 

 Babyroussa,^ e.g. : it usually coalesces with the squamosal, 27, as in 

 the Hog ; but retains its distinctness in the Echidna; its apophysial 

 character is usually well-marked, and it is known as the * mastoid 

 process ' in Anthropotomy. In most Mammals the pleurapophysis, 

 38, retains its primitive independency and rib-like form, with usually 

 the ^ head ' and * tubercle ; ' but by reason of its arrested growth it 

 has been called ^ styloid ' bone or process. Sometimes it is sepa- 

 rated from the short hasmapophysis, 40, by a long ligamentous 

 tract, sometimes is immediately articulated with it, or by an inter- 

 vening piece. The haemal spine, 4i, is usually small, and always 

 single. The rudiments of hypobranchial elements, 46, are retained 

 as diverging appendages of the parieto-hasmal arch in all Mam- 

 mals, and have received the special names of ' posterior cornua,' 

 or ^ thyrohyals,' from their subservient relationship to the larynx. 

 In the frontal segment, Nm, the centrum, 9, and neurapophyses, 10, 

 very early coalesce. Two separate osseous centres mark out the 

 body, and each neurapophysis has its distinct centre, the optic fora- 

 mina, op, being first surrounded by the course of the ossification 

 from these points. The superior developement of the neurapophy- 

 sial plates, 10, as compared with those of the parietal vertebra, 6, in 

 most Mammals, harmonises with the greater developement of the 

 prosencephalon ; but the chief bulk of this segment is protected 

 by the expanded spines of the frontal, 11, and parietal, 7, vertebra, 

 and the intercalated squamosal, 27. This appendicular piece not 

 only fulfils some of the functions of the proper cranial neurapo- 

 physes, but, likemse, the normal office of the frontal pleurapo23hysis, 

 28, in the support, viz., of the distal elements of the htemal 

 arch, 29-32, which now articulate directly with 27, in place of 28, 

 as in all oviparous Vetebrates. The true pleurapophysis of the 

 frontal vertebra, 23, is almost restricted in the Mammalian class to 

 functions in subserviency to the organ of hearing ; is sometimes, 

 as in the Hog, swollen into a large bulla ossea, like the parapo- 

 physes and pleurapophyses of the cervical vertebras of Cohitis ; is 

 sometimes produced into a long auditory tube, and somethnes 

 reduced to the ring supporting the tympanic membrane. Yet, 

 under all these changes, since its special homology is demonstrable 

 with 28 in the Bird, fig. 26, Turtle, fig. 91, Vol. I., and Crocodile, 

 fig. 92, Vol. I., as well as with the teleologically compound bone, 

 28, a, h, c, d, in the Fish, fig. 81, Vol. I., so likemse must its general 



^ XLiv. p. 556, no. 3338, in which the petrosal is instructively distinct from all the 

 surrounding vertebral elements composing the otocrane. 



