16 THE ANATOMY OP VERTEBEATED ANIMALS. 



a vertebral (V.r) and a sternal (St.r) part. The former re- 

 mains unossified for a considerable distance at its distal 

 end {V.r'); the latter is more or less converted into car- 

 tilage bone. The proximal end of the vertebral rib bifur- 

 cates into a tuherculum {t) and a capitulum [Cp). The distal 

 end of the sternal rib unites with the more or less ossi- 

 fied but imsegmented cartilage, which fomis the sternum 

 iSt). A cartilaginous, or partly ossified, uncinate process {P.u) 

 projects from the posterior edge of the vertebral I'ib, over 

 the intei'costal space. Tho student will find it convenient 

 to familiarise himself with the conception of such a spinal 

 segment as this, as a type, and to consider the modifica- 

 tions hereafter described with reference to it. 



In the majority of the Vertebrata, the caudal A'ertebrse 

 gradually diminish in size towards the extremity of the 

 body, and become reduced, by the non-development of osse- 

 ous processes or arches, to mere centra. But, in many fishes, 

 which possess well-ossified trunk vertebrae, no distinct 

 centra are developed at the extremity of the caudal region, 

 and the notochord, invested in a more or less thickened, 

 fibrous, or cartilaginous sheath, persists. Notwithstanding 

 this embryonic condition of the axis of the tail, the superior 

 and inferior arches, and the interspinous bones, may be 

 completely formed in cartilage or bone. 



Whatever the condition of the extreme end of the spine 

 of a fish, it occasionally retains the same direction as the 

 trunk part, but is far more generally bent up, so as to form 

 an obtuse angle with the latter. In the former case, the ex- 

 tremity of the spine divides the caudal fin-rays into two 

 nearly equal moieties, an upper and a lower, and the fish is 

 said to be diphycercal (Fig. 6, A). In the latter case, the 

 upper division of the caudal fin-rays is lauch smaller than 

 the lower, and the fish is heterocercal (Fig. 6, B, C). 



In most osseous fishes the hypural bones which support 

 the fin-rays of the inferior division become much expanded, 

 and either remain separate, or coalesce into a wedge- 

 shaped, nearly symmetrical bone, which becomes ankylosed 

 with the last ossified vertebral centram. The inferior fin- 



