XIII 



PHYLUM CHORDATA 421 



The sternum (Fig. 1020) consists of six segments or siernelroB, 

 the first, the manubrium sterni r presternum, is larger than the 

 rest, and has a ventral keel. With the last is connected a rounded 

 cartilaginous plate, the sriphisternum. 



The skull (Fig. 1019), 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 proportion to the rest, the orbits closely approxi- 

 mated, being separated only by a thin inter-orbital partition, and 

 the optic foramina united into one. But certain important differ- 

 ences 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 elements of the skull, 

 originally distinct, become completely fused together so that their 

 limits are no longer distinguishable. 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 

 straight, sometimes wavy, sometimes zig-zagged, serrations of the 

 edges of the two bones interlocking ; in some cases the edges of 

 the bones are bevelled off and the bevelled edges overlap, forming 

 what is termed a squamous suture. 



Another conspicuous difference between the skull of the Rabbit 

 and that of the Pigeon is in the mode of connection of the lower 

 jaw 7 , which in the former articulates directly with the skull, the 

 quadrate, through which the union is effected in the Pigeon, being 

 apparently absent. Certain large apertures which are distinguish- 

 able are readily identified with the large openings in the skull of 

 the Pigeon. In the posterior Avail of the skull is a large rounded 

 opening, the foramen magnum, flanked with a pair of smooth 

 rounded eleA T ations or condylcs for articulation with the first 

 vertebra, these obviously corresponding to the single condyle 

 situated in the middle below the foramen in the Pigeon. A large 

 opening, situated at the end of the snout and looking forwards, 

 obviously takes the place of the external nares of the Pigeon: 

 and a large opening in the roof of the mouth leading forward to 

 the external nasal opening, plainly represents, though much wider 

 and situated further back, the internal or posterior nares of the 

 Pigeon ; Avhile the rounded tubular opening (and. me.) situated at 

 the side of the posterior part of the skull, some distance behind the 

 orbit, is evidently the same as the auditory aperture of the Pigeon. 



Surrounding the large opening of the foramen magnum are the 

 bones of the occipital region of the skull, the supra-, ex- and lasi- 

 occipitats. The first of these (s. oc.) is a large plate of bone Avhose 

 external surface is directed backwards and upwards, and eleA^ated 

 in the middle into a shield-shaped prominence. The ex+occipitals 



