356 CLASSIFICATION OF VERTEBRATES. 



process of human anatomy) the latter with articular facets (re- 

 duced parapophyses) on the centra. The ribs usually present 

 bony and cartilaginous portions, the latter reaching the sternum. 

 In floating ribs the union with the sternum is lacking, while false 

 ribs are without vertebral connections. 



The sternum frequently retains throughout life the separate 

 elements or sternebrae of which it is composed ; but these may 

 fuse into an elongate plate, the corpus sterni or mesosternum, 

 with an anterior portion, the manubrium or presternum, and a 

 posterior xiphisternum or ensiform process with which no ribs 

 articulate. The episternum (p. 149), which is laid down in carti- 

 lage, is placed in front of the sternum, but retains its distinctness 

 only in the monotremes and some edentates and rodents. In 

 other forms it fuses with the sternum. 



In the mammalian skull there is a more intimate relationship 

 between the cranial and facial elements than is the case in the 

 lower vertebrates. There is also a marked tendency to the 

 fusion of bones which are distinct in the lower vertebrates, but 

 usually the process of co-ossification is not complete except in the 

 hyoid and lower jaw, many of the bones being suturally united 

 throughout life. 1 The floor of the skull is preformed in car- 

 tilage, its roof of membrane bones. No interorbital septum 

 occurs, its remnants being found in the crista galli process of the 

 ethmoid, while the lateral walls continue forward to the ethmoid 

 region. Basi- and presphenoids frequently fuse, and from the 

 sphenoid thus formed a greater and a lesser wing arises on either 

 side, these being the ali and orbitosphenoids respectively of the 

 lower forms. The pterygoids also unite with the sphenoids, 

 forming the pterygoid processes. 



Basi-, ex-, and supraoccipitals may remain distinct, or they 

 may fuse, sometimes late in life, into a single occipital bone 

 which bears a pair of occipital condyles arising from the exoccip- 

 itals, the basioccipital but rarely contributing to their formation. 



The sides of the skull are formed in part by the sphenoidal 

 alae, in part by the temporal bones, each of which is a complex 

 of several elements, the petrosal (fused pro-, epi-, and opi- 



1 The obliteration of sutures has progressed the farthest in monotremes, the weasel 

 and some apes. 



