496 ZOOLOGY SECT. 



sacrum consisting of closely united vertebrae, the number of which 

 varies in the different orders. The caudal region varies greatly 

 as regards the degree of its development. In the caudal region of 

 many long-tailed Mammals there are developed a series of chevron, 

 bones V-shaped bones, which are situated opposite the inter- 

 vertebral spaces. 



The centrum of each vertebra ossifies from three centres 1 a 

 middle one, an anterior, and a posterior. The middle centre forms 

 the centrum proper ; the anterior and posterior form the epiphyses. 

 The epiphyses are almost entirely absent in the Monotremes, and 

 in the Dugong (Sirenia) have not been detected. Between successive 

 centra are formed a series of discs of fibre-cartilage the inter- 

 vertebral discs represented in lower Vertebrates only in Crocodiles 

 and Birds. The anterior and posterior surfaces of the centra are 

 nearly always flat. 



The sternum consists of a number of segments the presternum 

 in front, the mesosternum, or corpus sterni, composed of a number 

 of segments or sternebrce, in the middle, and the xiphisternum behind. 

 The sternum is formed in the foetus in great part by the separating 

 off of the ventral ends of the ribs. Some of the Cetacea and the 

 Sirenia are exceptional in having a sternum composed of a single 

 piece of bone. The sternal ribs, by which the vertebral ribs are 

 connected with the sternum, are usually cartilaginous, but frequently 

 undergo calcification in old animals, and in some cases early become 

 completely converted into bone. 



The skull of a Mammal (Fig. 1137) contains the same chief 

 elements and presents the same general regions as that of the 

 Sauropsida, but exhibits certain special modifications. A number 

 of the bones present in the skull of Sauropsida are not represented, 

 or, at all events, not certainly known to be represented definitely 

 by separate ossifications in the Mammalia. Such are the supra- 

 orbital, the pre-frontal, the post-orbital, the ecto-pterygoid and 

 the quadrato-jugal. The bones of the skull, with the exception of 

 the auditory ossicles, the lower jaw, and the hyoid, are all 

 immovably united together by means of sutures. 



The palatine bones develop palatine plates separating off a 

 posterior nasal passage from the cavity of the mouth, a condition 

 found among the Sauropsida only in the Crocodilia, and, to a less 

 extent, in the Chelonia and some Lizards. 



The zygomatic arch is a strong arch of bone formed partly of 

 the squamosal, partly of the jugal, and partly of the maxilla : in 

 position it represents the upper temporal arch of Amphibia and 

 Sauropsida, but is differently constituted (see p. 339). The orbit 

 in the skull of some Mammals is completely enclosed by bone, 

 constituting a well-defined cavity ; in others it is not completely 



1 Usually the two centres of ossification which form the neural arches also 

 contribute to the formation of the bony centrum. 



