xm PHYLUM CHORDATA 81 



In all classes above Fishes the paired fins are, as we have seen, 

 replaced by five-toed or pentadactyle limbs. These are supported by 

 bones, probably to be looked upon as greatly modified pterygio- 

 phores, and obviously serially homologous in the fore- and hind-limbs, 

 in the proximal division of each limb there is a single rod-like bone, 

 the humcrus (Fig. 776, HU), or upper-arm-bone, in the fore-limb, 

 the femur (Fig. 777, FE), or thigh-bone, in the hind-limb. In the 

 middle division there are two elongated bones, an anterior, the 

 radius (RA), and a posterior, the ulna (UL), in the fore-limb ; an 

 anterior, the tibia (TI), and a posterior, the fibula (FI), in the hind- 

 limb. Next follow the bones of the hand and foot, which are again 

 divisible into three sets : carpals or wrist-bones, metacarpals (mtcp) 

 or hand-bones, the phalanges (ph) or finger-bones, in the fore- 

 limb ; tarsals or ankle-bones, metatarsals (mtts) or foot-bones, and 

 phalanges (ph) or toe-bones, in the hind-limb. The carpals and 

 tarsals consist typically of three rows of small nodules of bone or 

 cartilage, the proximal row containing three, the middle two, and 

 the distal five elements. The three proximal carpals are called 

 respectively radiale (ra), intermedium (int), and ulnare (ul), those 

 of the middle row the first and second centralia (en. 1, en. 2), those 

 of the third row the five distalia (dst. 1-5), the separate elements 

 being distinguished by numbers, counting from the anterior or 

 radial edge of the limb. In the tarsus the bones of the first row 

 are known respectively as tibiale (ti), intermedium (int), and 

 f bul ire (fi), those of the second row as centralia (en. 1, en. 2), and 

 those of the third as distalia (dst. 1-5). The metacarpals 

 (mtcp. 1-5) and metatarsals (mtts. 1-5) are five rod-like bones, 

 one articulating with each distale : they are followed by the 

 phalanges (ph), of which each digit may have from one to five. 

 The first digit of the fore-limb (Fig. 776, 7) is distinguished as the 

 pollex or thumb, that of the hind-limb (Fig. 777, I) as the hallux 

 or great toe ; the fifth digit of each limb ( V) is the minimus. 



In connection with the paired appendages are formed supporting 

 structures called the limb-girdles ; they occur in the portions of the 

 trunk adjacent to the appendages and serve for the articulation of 

 the latter. In the embryonic condition they are continuous with 

 the basalia and are probably to be looked upon as ingrowths of 

 the primitive fin-skeleton (Fig. 775). The shoulder-girdle or pectoral 

 arch has primarily the form of paired bars, which may unite in 

 the middle ventral line so as to form an inverted arch. Each bar 

 i.e., each half of the arch furnishes a concave or convex glenoid 

 surface (Fig. 776, gl.) for the articulation of the pectoral fin or fore- 

 limb, and is thereby divided into two portions a dorsal or 

 scapular region, above the glenoid surface, and a ventral or coracoid 

 region below it. The coracoid region is again divisible, in all classes 

 above Fishes, into two portions : an anterior, the pro-coracoid 

 (p. cor.), and a posterior, the coracoid proper. Each of these regions 



