ORNITHORHYNCHIDAZ 119 
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 vertebre 
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 
fosse. 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 
supportsin theadult 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-czecal valve, 
or marked distinction between large and small intestine, but has a 
small, slender vermiform cecum 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 représentatives 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 ORNITHORHYNCHIDA. 
Ornithorhynchus.\—Cerebral hemispheres smooth. Premaxille 
and mandible expanded anteriorly and supporting a horny beak 
1 Blumenbach, Voigts Magazin, vol. ii. p. 205 (1800). 
