J 76 THE ANATOMY OF VERTEBRATED ANIMALS. 



appendages ; but the embryo is born in an imperfect condition, 

 and may be provided with a knob or caruncle upon the pre- 

 maxillse, such as is found in the Sauropsida. In the adult 

 the heart exhibits a fossa ovalis. 



Both genera of the Ornithodelphia are restricted to Aus- 

 tralia, including Tasmania under that name. 



The one of them, Echidita, has the body covered with 

 spines, like a porcupine. It possesses strong digging feet, 

 and a narrow, toothless mouth, from which the long tongue, 

 with which it licks up the ants upon which it preys, is pro- 

 truded. 



The other genus, Ornithorhynchus, has soft fur ; a flattened 

 muzzle resembling the beak of a duck, and covered with a 

 leathery integument ; and clawed, but strongly webbed feet, 

 fitting it for its altogether aquatic mode of life. The Orni- 

 thorhynchus, in fact, frequents fresh-water pools and rivers, 

 very much like a water-rat, sleeping and breeding in burrows 

 excavated in the bank. 



In these animals the angle of the mandible is not inflected. 

 They are devoid of any external ear ; and, in the males, a kind 

 of spur, which is perforated, and gives exit to the secretion of 

 a gland, is attached to the astragalus. The function of this 

 organ is unknown. In each genus the heart is provided with 

 two superior cavse. In Echidna the right auriculo-ventricle 

 valve is membranous, but, in Orniihorhynchus, it is more or 

 less fleshy. 



The hemispheres of the brain are abundantly convoluted 

 in Echidna, but are smooth in Ornithorhynchus. The ovaries 

 are of equal size in Echidna; but, in Ornithorhynchus, the 

 right is much smaller than the left, as in Birds. As has al- 

 ready been stated. Echidna is entirely devoid of teeth, while 

 Ornithorhynchus has four large horny teeth. 



n. The DiDELPHiA. — In the Didelphia, the " odontoid 

 process " early becomes completely anchylosed with the body 

 of the second vertebra; and, usually, all the cervical ribs 

 speedily lose their distinctness, as in Mammals in general. 



The coracoid is reduced to a mere process of the scapula 

 and does not come near the sternum. There is no epicora- 

 coid, such as exists in the Ornithodelphia. There is no T- 

 shaped interclavicle, but the clavicles, which are always pres- 

 ent (except in PerameUs) articulate with the manubrium of 

 the sternum, in the same way as in ordinary Mammalia. 

 The floors of the acetabula are completely ossified, and consc' 



