XIII PHYLUM CHORDATA 511 



successive centra are formed a series of discs of fibro-cartilage — 

 the intervertchral discs — represented in lower Vertebrates only in 

 Crocodiles and Birds. The anterior and posterior surfaces of the 

 centra are nearly always flat. 



The stermtm consists of a number of segments — the 2J'>'6Sternu7n 

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

 of segments or sternebrce, in the middle, and the xi2)Msternum 

 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 cartila- 

 ginous, but frequently undergo calcification in old animals, and in 

 some cases early become completely converted into bone. 



The shvll of a Mammal (Fig. 1125) contains the same chief 

 elements^ancT 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. True pitcrygoids are 

 only known to occur in the Monotremes, the elements to which 

 that name is usually given in other Mammals apparently rejDre- 

 senting a part of the parasphenoid. 



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. 349). The orhit 

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

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

 surrounded by bone behind, and so communicates freely with the 

 temporal fossa, which lies behind it. 



The periotic bones {pro-otic, opisthotic, and epi-otic) become 

 completely fused together in the skull of Mammals. Part of the 

 periotic mass sometimes projects on the exterior at the hinder 

 part of the lateral region of the skull, and is the mastoid portion ; 

 the rest is commonly called the petrous portion of the periotic, 

 and encloses the parts of the internal ear — the mastoid portion 

 containing only air-cells. The tympanic bone, which perhaps 

 represents the quadrato-jugal of Sauropsida, sometimes forms a 

 long tube, sometimes a mere ring of bone ; in other cases it not 



