320 THE ANATOMY OF VERTEBEATED ANIMALS. 



3. Edentata. 



b. Median incisor teeth are almost always developed in 

 one or both jaws. 

 i. The uterus developes no decidua {Non-deciduata'). 



4. Unqvlata. 



5. Toxodontia (?).* • 



6. Sirenia (?).* 



7. Cetacea. 



ii. The uterus developes a decidua (^Decidnata). 

 a. The placenta is zonary. 



8. Hi/raroidta. 



9. Prohoscidea. 



10. Carniiora. 



B. The placenta is discoidal. 



11. Rod.Mtia. 



12. Insect ivora. 



13. Clieiroptera. 



14. Primates. 



I. Tlie Oenithodelphia are those mammals whicli 

 approacli nera-est to the Smiropsida, although se^Darated 

 from them by all the essential characters of the Mammalia 

 which have already been defined. 



The two genera Echidna and Ornithorynchus, which con- 

 stitute this division, agree with one another, and differ from 

 all other Mammals, in the combination of the following 

 characters : — 



In the spinal column, the centra of the vertebrae are 

 devoid of epiphyses. The os odontoideum, or so-called 

 " odontoid process" of the second cei-vical vertebra remains 

 for a long time, if not throughout life, unankylosed with 

 the body of that vertebra, as is the case in many Reptiles. 

 And some of the cervical ribs, in like manner, long persist 

 in a separate condition. 



A striking Sauropsidan and Amphibian feature, peculiar 

 to the Ornithodelphia, is seen in the fact that the coracoid, 

 which is a large bone, articulates with the sternum directly. 

 In front of it is another considerable ossification called the 

 epicoracoid, which corresponds in position, though not in 

 the manner of its ossification, with the ossified cartilage so 

 termed in reptiles. In these mammals alone, again, there 

 is a T-shaped interclavicle, which supports the clavicles. 



* The placentation of the Toxodontia and Sirenia is unknown. 



