234 OUTLINES OF EVOLUTIONARY BIOLOGY 



a 



'Ep.C. 



including suprascapula, scapula, coracoid, epicoracoid, clavicle 

 and interclavicle. 



The mammals, on the other hand, are characterized by the 

 possession of an epidermic covering of hairs ; they generally 

 have very minute eggs almost destitute of yolk, and they always 

 suckle their young ; they usually have no cloaca but separate 

 openings for the alimentary canal and urino-genital ducts, and 

 the shoulder girdle (Fig. 90) is very greatly reduced, being often 

 represented by only a single bone, the scapula, with a small 



remnant or vestige of 

 the coracoid completely 

 fused with it, though a 

 slender clavicle is some- 

 times present as well. 



There exists in Aus- 

 tralia and some of the 

 adjacent islands, how- 

 ever, a small but very 

 interesting group of 

 animals known as the 

 Monotremata, repre- 

 sented by the duck-billed 

 Platypus (Ornithorhyn 

 chus, Fig. 91) and the 



spiny anteater (Echidna, 

 FIG. 89. Shoulder Girdle and Sternum of a 



Lizard ( Varanus) seen from below. 



fnt 



Cl. Clavicle; Cor. Coracoid; Ep. C. Epicoracoid; 

 Gl. Glenoid cavity ; Int. Interclavicle ; E. Ribs ; 

 Sc. Scapula; St. Sternum. 



Fig. 92). The fact that 

 these animals possess 

 hair and suckle their 

 young justifies us in 

 classifying them amongst the Mammalia, but at the same time 

 they exhibit certain characters in respect of which they differ 

 from all typical mammals and agree with the reptiles. Thus they 

 lay large, heavily-yolked eggs, 'the alimentary canal and urino- 

 genital ducts discharge into a common cloaca, and the shoulder 

 girdle (Fig. 93) is made up of a considerable number of separate 

 bones almost exactly as in reptiles, most remarkable amongst 

 which is the interclavicle, which is found in no other mammals. 



Here then we have animals still existing which undoubtedly 

 occupy an intermediate position between the reptiles and the 

 typical mammals, and the three groups exhibit a progressive series 

 of structural peculiarities. Such definite connecting links as the 



