MAMMALIA. ORNITHODELPHIA. 



347 



the central portion of the acetabulum is unossified as in birds. 

 The humerus has a prominent deltoid crest ; its ends are much 

 expanded, and the distal end is pierced by an ent-epicondylar 



FIG. 66. VENTRAL VIEW OF THE SHOULDER-GIRDLE AND STERNUM OF 

 A DUCKBILL (Ornithorhynchus paradoxus) x f (after PARKER). 



1 and 2. scapula. 8. presternum. 



3. coracoid. 9. third segment of mesoster- 



4. precoracoid (epicoracoid). num. 



5. glenoid cavity. 10. sternal rib. 



6. interclavicle. 11. intermediate rib. 



7. clavicle. 12. vertebral rib. 



foramen. The fibula has a broad proximal process resembling 

 an olecranon. The limbs and their girdles bear a striking 

 resemblance to those of some Theromorphous reptiles. 



The order Monotremata includes only two living families, 

 the Echidnidae and Ornithorhynchidae. 



