342 THE ANIMAL KINGDOM. 



anterior edge of both scapula and coracoid may, when large, be partially 

 converted into membranous spaces with intervening processes, prae- and 

 meso-scapula ; prae- and meso-coracoid. The two clavicles in some 

 Amphibia and in higher types unite ventrally in the embryo, and from 

 their point of union is differentiated an interclavicle or episternum. This 

 episternum is free in Reptilia when it is present. It unites in Aves with 

 the costal sternum forming the keel (?). In Mammalia except Prototheria, 

 where it is large and free, it fuses with the manubrium sterni, and then 

 the clavicular region of this bone either persists, aborts, or is transformed 

 into ligament. The origin of the epipubic cartilage, which often exists 

 projecting forwards from the median anterior edge of the pubes, is 

 unknown. The two pubes and ischia unite ventrally, and the line of 

 union is termed the symphysis. 



The limbs articulate with cup-shaped cavities the glenoid cavity for 

 the fore- and the acetabulum for the hind-limb in all classes higher than 

 Pisces. In the latter tubercles may take the places of cavities. In 

 Amphibia and upwards the fore-limb is divisible into a series of segments 

 pre-formed in cartilage, but all ossifying viz., a humerus, articulating with 

 the shoulder-girdle ; a radius and ulna, articulating with the humerus and 

 with each other ; followed by a carpus, consisting of (typically) a radiale = 

 scaphoid, intermedium = lunar, and ulnare = cuneiform, constituting a 

 proximal row ; a centrale ; and then a distal row composed of carpale 

 i = trapezium, 2 = trapezoid, 3 = os magnum, carpalia 4 and 5, or 4 + 5 

 = unciform, to which articulate the digits typically 5 in number, composed 

 each of a proximal metacarpal, and a distal series of phalanges. The 

 corresponding parts in the hind-limb are femur ; tibia, and fibula ; tarsus 

 composed of tibiale, intermedium, or both together = astragalus, fibulare 

 = calcaneum ; centrale = navicular ; tarsale i = ento-cuneiform, 2 = meso- 

 cuneiform, 3 = ecto-cuneiform, tarsalia 4 and 5, or 4 + 5 = cuboid ; and 

 5 digits each with a meta-tarsal and phalanges. There are great modi- 

 fications observable in the hand and foot of different groups of Vertebrata 1 . 

 In addition to the paired limbs, the Ichthyopsida possess a system of 

 azygos fins, either temporarily or permanently. An account of them is 

 given under the general introduction to that division. 



In addition to the above-named bones, ossifications appear frequently 



1 The account given in the text is the one ordinarily accepted. In some Urodele Amphibia the 

 centrale is double. A digit external to the great toe also occurs in some of them, and some Anuran 

 Amphibia. Baur (Z. A. viii. 1885) has come to the conclusion that the astragalus of Mammalia 

 represent one of the two centralia + the intermedium, the tibiale being represented by a sesamoid, 

 or forming part of the navicular (= the other centrale). The unciform and cuboid represent the 

 fifth carpal and tarsal respectively. Traces of a finger external to the thumb, and of a toe external 

 to the great toe, are found in some Mammals, the sesamoid of the abducens pollicens representing 

 the former, an extra ossicle seen in many Carnivora the latter. Cf. Id. on Archegosaurus, Z. A. ix. 

 1886. 



