376 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- 

 maxillae, such as is found in the SauropsidM, In the adult 

 the heart exhibits n. fossa ovalis. 



Both genera of the Ornitliodelphia are restricted to Aus- 

 tralia, including Tasmania under that name. 



The one of them, Echidna^ has the body covered ^Yith 

 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, OrnitJiorhyncJius, 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 O^mi- 

 thorhynchiis^ 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 astragalu,s. The function of this 

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

 two superior cavae. In Echidna the right auriculo- ventricle 

 valve is membranous, but, in Ornithorhynchus^ 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. 



II. 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 ej^icora- 

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

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

 ent (except in Perameles) articulate with the manubrium of 

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

 The floors of the acetabula are completely ossified, and conse- 



