ORNITHORH YNCHID^E 1 1 9 



indicative of a very marked separation of neck from thorax, not 

 seen in the existing Sauropsida. The upper ends of the ribs 

 are attached to the sides of the bodies of the dorsal vertebrae 

 only, and not to the transverse processes. The sternal ribs are 

 well ossified, and there are distinct partly ossified intermediate ribs. 

 The cerebral cavity, unlike that of the lower Marsupials or the 

 Reptiles, is large and hemispherical, flattened below and arched 

 above, and about as broad as long. The cribriform plate of the 

 ethmoid is nearly horizontal. The cranial walls are very thin, and 

 smoothly rounded externally, and the sutures become completely 

 obliterated in adult skulls, as in Birds. The broad occipital region 

 slopes upwards and forwards, and the face is produced into a long 

 and depressed rostrum. The bony palate is prolonged backwards, 

 so that the posterior nares are nearly on a level with the glenoid 

 fossae. The mandible is without distinct ascending ramus ; the 

 coronoid process and angle are rudimentary, and the two halves are 

 loosely connected at the symphysis. The fibula has a broad, 

 flattened process, projecting upwards from its upper extremity 

 above the articulation, like an olecranon. In the male there is an 

 additional, flat, curved ossicle on the hinder and tibial side of the 

 plantar aspect of the tarsus, articulating chiefly to the tibia, which 

 supports in the adult a sharp-pointed perforated horny spur, with which 

 is connected the duct of a gland situated beneath the skin of the back 

 of the thigh, the function of which is not yet clearly understood. (A 

 rudimentary spur is found in the young female Ornithorhynchus, but 

 this disappears when the animal becomes adult.) The stomach is 

 sub-globular and simple ; the alimentary canal has no ileo-caecal valve, 

 or marked distinction between large and small intestine, but has a 

 small, slender vermiform caecum with glandular walls. The liver 

 is divided into the usual number of lobes characteristic of the 

 Mammalia, and is provided with a gall-bladder. 



In the presence of three distinct bones developed from cartilage 

 in the shoulder-girdle (viz. scapula, coracoid, and pre- or epi-coracoid) 

 the Monotremes agree with the Anomodont reptiles (see p. 83), 

 and with no other representatives of that class. The precoracoid 

 of the Anomodonts is, however, distinguished by extending upwards 

 to articulate with the acromial process of the scapula. The 

 Monotreme humerus is, moreover, strikingly like the corresponding 

 bone of many of the Anomodonts and of some of the allied 

 Labyrinthodont Amphibians. 



Family ORNITHORHYNCHID^E. 



Ornithorhynchus. 1 Cerebral hemispheres smooth. Premaxillae 

 and mandible expanded anteriorly and supporting a horny beak 

 1 Blumenbach, Voigts Magazin, vol. ii. p. 205 (1800). 



