XII PHYLUM CHORDATA 495 



ally diminish in size, the most posterior vertebra being 

 represented merely by nearly cylindrical centra. 



There are twelve pairs of ribs, of which the first seven are 

 true ribs, i.e. are connected by their cartilaginous sternal 

 ribs with the sternum ; while the remaining five, the so- 

 called false or floating ribs, are not directly connected with 

 the sternum. All, except the last four, bear two articular 

 facets, one on the vertebral extremity or capitulum, and the 

 other on a little elevation or tubercle situated at a little dis- 

 tance from this, the former for the bodies, the latter for the 

 transverse processes of the vertebras. 



The sternum (Fig. 300) consists of six segments or sterne- 

 brx. The first or manubrium sterni or presternum is larger 

 than the rest, and has a ventral keel. With the last is con- 

 nected a rounded cartilaginous plate, the xipldsternum. 



The skull (Fig. 299), if we leave the jaws out of account, 

 is not at all unlike that of the pigeon in general shape. 

 The length is great as compared with either the breadth or 

 the depth; the maxillary region, or region of the snout 

 (corresponding to the beak of the pigeon), is long in pro- 

 portion to the rest, the orbits closely approximated, being 

 separated only by a thin inter-orbital partition, and the optic 

 foramina united into one. But certain important differences 

 are to be recognised at once. One of these is in the mode 

 of union of the constituent bones. In the pigeon, as we 

 have seen, long before maturity is attained, the bony ele- 

 ments of the skull, originally distinct, become completely 

 fused together so that their limits are no longer distinguish- 

 able. In the rabbit, on the other hand, such fusion between 

 elements only takes place in one or two instances, the great 

 majority of the bones remaining distinct throughout life. 

 The lines along which the edges of contiguous bones are 

 united — the sutures as they are termed — are sometimes 



