VERTEBRATA. 34! 



and are probably independent formations, but in some Pisces they are 

 continuous at an early period of development with the haemal arches. 

 They always ossify independently. At their dorsal extremities they articu- 

 late with the haemal arch, or the transverse processes, or with the 

 vertebral centrum, as well as with a transverse process. The double 

 articulation in Urodele Amphibia is brought about by the addition to 

 the primitive rib of a dorsal rod which fuses with it and is thus connected to 

 the transverse process of the neural arch. The persistence of this dorsal 

 element, its loss, or the loss of the ventral articulation, is sufficient to 

 explain the variations observable in the way the ribs articulate with the 

 vertebrae in higher Vertebrata. In Amniota the ventral ends of the ribs 

 meet in the middle line. A ventral segment is cut off from each rib, and 

 from the elements thus derived the sternum (costal sternum) is formed. 

 The remainder of the rib is generally divided into a vertebral and a sternal 

 section, with an intercalated ' intermediate rib ' in some Reptilia. 



All Vertebrata possess typically two pairs of limbs the pectoral and 

 pelvic, one or the other, or both of which are sometimes aborted. It is 

 still doubtful however if they were ever developed in the ancestors of the 

 Cyclostomi. They appear to have arisen at first as a continuous ridge 

 on each side of the body. A trace of -this origin is seen in the ontogeny 

 of Elasmobranchii. The primitive position of the limbs is one of hori- 

 zontal extension, but it is only incompletely retained in Elasmobranchii 

 and not at all elsewhere. The skeleton of such a primitive limb appears as 

 a basal bar or plate of cartilage, the outer edge of which segments into 

 radially placed radialia. These are borne upon the rest of the bar which 

 divides into two or three large pieces pro-, meso-, and meta-pterygium, 

 or the two latter only. The radialia are bordered by an integumental fold, 

 supported by fine rays similar to those of the azygos fins. In connection 

 with the horizontal bar of cartilage in each limb is a vertical rod of 

 cartilage possibly derived in the first instance from the horizontal bar. 

 This vertical rod forms the shoulder girdle for the fore-limbs, the pelvic 

 for the hind. The shoulder girdle in Pisces becomes connected in most 

 cases with investing bones. It is always divisible into a portion dorsal 

 to the articulation of the limb known as the scapula and a portion ventral 

 to it, the coracoid. A clavicular process grows forwards from the junction 

 of the two regions, or in higher forms from the scapula. The corresponding 

 portions of the pelvic girdle are the ilium (dorsal), the pubes, and ischium 

 (ventral) ; the pubes perhaps homologous with the clavicle. All these 

 parts ossify independently. A supra-scapula or vertebral border may 

 either remain cartilaginous at the dorsal end of the scapula or ossify 

 separately from it ; and an epi-coracoid may be similarly formed from 

 the ventral or sternal edge of the coracoid which is in contact with the 

 costal sternum except in the Eu- and Meta-theria among Mammalia. The 



