346 THE VERTEBRATE SKELETON. 



Of these the calcaneum is the fibulare, and the astragalus is 

 generally regarded as the tibiale and intermedium fused 1 . 



Subclass I. ORNITHODELPHIA OR PROTOTHERIA. 



This sub-class contains only a single order, the Monotre- 

 mata, and the following characteristics are equally applicable 

 to the subclass and to the order. The vertebral centra have 

 no epiphyses, and the odontoid process remains for a long time 

 free from the centrum of the second vertebra. With the 

 exception of the atlas of Echidna the cervical vertebrae are 

 without zygapophyses. The cranial walls are smooth and 

 rounded, and the sutures between the several bones early 

 become completely obliterated as in birds. The mandible is a 

 very slight structure, with no ascending ramus, and with the 

 coronoid process (see p. 398) and angle rudimentary. The 

 auditory ossicles show a low state of development. The tuber- 

 cula of the ribs articulate with the sides of the centra of the 

 thoracic vertebrae, not with the transverse processes. Some of 

 the cervical ribs remain for a long time separate from the verte- 

 brae. Well ossified sternal ribs occur. No true teeth are present 

 in the adult. The young Ornithorhynchus has functional molar 

 teeth, but in the adult their place is taken by horny plates. 

 In the Echidnidae neither teeth nor horny plates occur. 



The coracoid (fig. 66, 3) is complete and well developed, 

 and articulates with the sternum. A precoracoid (epicoracoid) 

 occurs in front of the coracoid, and there is a large interclavicle 

 (fig. 66, 6). The ridge on the scapula, corresponding to the 

 spine of other mammals, is situated on the anterior border 

 instead of in the middle of the outer surface. Epipubic bones 

 are present. In the Echidnidae, but not in Ornithorkynchus*, 



1 Baur, however, suggests (Anat. Anz. vol. iv. 1889), that a tibial 

 sesamoid found in Procavia, many rodents, edentates and Ornithorhyn- 

 chus is a vestigial tibiale, and that the astragalus is the intermedium. 



2 This perforation of the acetabulum in Echidna is a secondary 

 character occurring late in development, and consequently is not of 

 phylogenetic importance. 



