THE DIDELPHIA. 323 



heaii is iDrovided vnth two superior cav^. In Echidna tlie 

 right auriculo-ventricle valve is membranous, but, in Or- 

 nithorhynchus, 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 already been stated. Echidna is entirely devoid of teeth, 

 while Ornithorhynchus has four large homy teeth. 



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

 process" early becomes completely ankylosed with the body 

 of the second vertebra; and, usually, all the ceiwical 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 epi- 

 coracoid, such as exists in the Ornithodelphia. There is no 

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

 present (except in Perameles) articulate with the manu- 

 brium of the sternum, in the same way as in ovdmar j Mam- 

 malia. The floors of the acetabula are completely ossified, 

 and consequently are imperforate in the dry skeleton. The 

 cochlea is coiled upon itself. 



There is a ghallow cloaca, the sphincter muscle being 

 common to the urinary and genital apertures, but there is 

 no such urogenital chamber as in the Monotremata. The 

 ureters open directly into the bladder. 



In the male, the urogenital part of the urethra, and that 

 which traverses the penis, form one continuous canal, which 

 opens outwards only at the extremity of the penis. 



In the female, the vaginal, is perfectly distinct from the 

 lu-inary, passage. The mouths of the Fallopian tubes are 

 fimbriated, and the ova are not larger than those of the 

 MonodeJ2yhia. 



The mammary glands are provided with long teats. 



In all the preceding characters the Didelphia agree with 

 the Monodelphia, and difi"er from the Ornithodelphia. 



But they agree with the Ornithodelphia, and differ from 



