xiii PHYLUM CHORDATA 77 



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 tilnale (B, ti), intermedium 

 (int), and fibulare (fi), those of the second row as centralia (en. 1, 

 en. 2), and those of the third as distalia (dst. 1-5). The meta- 

 earpals (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 (A, i) is distinguished as the 

 pollen or thumb, that of the hind-limb (B, i) as the Jiallux 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 in-growths 

 of the primitive fin-skeleton (Fig. 721). 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 

 glcnoid surface (Fig. 722, A, 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 procora- 

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

 regions commonly ossifies, a cartilage bone, the scapula (SOP), 

 appearing in the scapular region, another, the coracoid (COR), in the 

 coracoid region, while in relation with the procoracoid is formed a 

 bone, the clavicle (CL), largely or entirely developed from 

 membrane. 



The constitution of the hip-girdle, or pelvic arch, is very similar. 

 It consists originally of paired bars, which may unite in the 

 middle ventral line, and are divided by the acetal)ulum (B, actb.\ 

 the articular surface for the pelvic fin or hind limb, into a dorsal 

 or iliac 'region, and a ventral or pubo-ischial region, the latter 

 being again divisible, in all classes above Fishes, into an anterior 

 portion, or piibis, and a posterior portion, or ischium. Each region 

 is replaced in the higher forms by a cartilage bone, the pelvic 

 girdle thus consisting of a dorsal ilium (IL) serially homologous 

 with the scapula, an antero-ventral pubis (PU) with the pro- 

 coracoid and clavicle, and a postero-ventral ischium (is) with 

 the coracoid. The long bones of the limbs are divisible each into 

 a shaft, and proximal and distal extremities. When ossification 

 takes place the shaft is converted into a tubular bone the 

 cartilaginous axis of which is absorbed and replaced by a vascular 



