370 THE ANIMAL KINGDOM. 



Paratherium, an extinct genus of Didelphidae, occurs in European Eocene and 

 upper Miocene strata. 



Waterhouse, Marsupiata, 'Natural History of Mammalia,' i. London, 1846; 

 Gould, 'Mammals of Australia,' 3 vols. London, 1845-63. 



Corpus callosum, Flower, Ph. Tr. 155, 1865. Tongue of Marsupialia, Poulton, 

 P. Z. S. 1883, and Q. J. M. xxiii. 1883. Union of crura penis to pelvis, Sack, Z. A. 

 ix. 1886. Female genitalia, Brass, Inaugural Dissertation, Leipzig, 1880, cf. 

 Zeitschr. f. ges. Naturw. liii. 1880, p. 672. Ovum, Poulton, Q. J. M. xxiv. 1884. 

 Foetal membranes, Osborne, Q. J. M. xxiii. 1883; Caldwell, Ibid. xxiv. 1884. 



SUB-CLASS PROTOTHERTA. 



THIS sub-class is represented by the single order Monotremata with 

 two families Ornithorhynchidae and Echidnidae. The former contains the 

 single genus Ornithorhynchus found in the rivers of Australia and Tasmania, 

 the latter the genus Echidna found in the same places, as well as in New 

 Guinea, and the genus Pro-echidna from the last-named locality. Ornitho- 

 rhynchus feeds on soft organisms inhabiting mud, &c. : its jaws are shaped 

 like a Duck's bill and covered by thickened epidermis, in which are lodged 

 peculiar tactile organs. The Echnidae have spines mingled with the hairs. 

 They feed on Ants, &c., which they catch with their long extensile tongue. 

 The sub-maxillary gland is of unusual size in this family. 



The cranial sutures close, and the surface of the cranium is polished as 

 in Aves. The rami of the lower jaw do not form a symphysis, and have 

 no ascending portion. The cervical ribs are distinct up to a certain age at 

 least : the odontoid process of the axis is long, and not fused to the centrum 

 of that vertebra. There are no epiphyses to the centra of the vertebrae 

 as in Sirenia among Eutheria. There are intermediate ribs as in many 

 Reptilia. The spine of the scapula forms the anterior border of that 

 bone : the coracoids reach the praesternum, and there are large distinct 

 ossified and overlapping epicoracoids and a T-shaped interclavicle. The 

 axis of the ilia is more vertical than usual among Mammals, and the ischia 

 meet in a ventral symphysis of great extent in Ornithorhynchus. The 

 centre of the acetabulum is fibrous in Echidna as in Aves. There are 

 epipubic bones. The fibula has a process homologous with the olecranon. 

 The os calcis is feebly developed, and the males have a curved spur per- 

 forated by the duct of a gland, and borne by an accessory ossicle on the 

 inner side of the tarsus. The spur is present also in the female, but re- 

 mains rudimentary, and is lost in aged specimens. 



The brain has a small corpus callosum and a large anterior commissure. 

 The olfactory nerve quits the skull in Ornithorhynchus as in lower Verte- 

 brata, in a single strand, and there is no lamina cribrosa. The cochlea is 

 curved, not spirally twisted. Ornithorhynchus has eight horny epidermic 

 teeth : the Echidnidae are edentulous. The right auriculo-ventricular valve 

 has no septal flap, or sometimes in Ornithorhynchus a rudimentary one. 



