2-74 THE ANATOMY OF VERTEBRATED ANIMALS. 



i. The uteru3 develops no decidua {Non-dedduata). 



4. JJiigidata. 



6. Toxodontia (?).* 



6. Sirevia (?).* 



Y. Ceiacea. 

 a. The uterus develops a decidua [Deciduaia). 

 u. The placenta is zonary. 



8. Hyracoidea. 



9. Prohoscidea. 



10. Oaryiivora. 



$. The placenta is discoldal. 



11. Rodeniia, 



12. Insedivora. 



13. Cheiroptera. 



14. Primates. 



L The Okmithodblphia are those Mammals which approach 

 nearer to the Sauropsida, although separated from them by all 

 the essential characters of the Mammalia which have already 

 been defined. 



The two genera Echidna and Ornithorhynchus, which con- 

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

 all other Mammals, in the combination of the following char- 

 acters : 



In the spinal column, the centra of the vertebrae are devoid 

 of epiphyses. The os odo7itoideum, or so-called "odontoid 

 process " of the second cervical vertebra remains for a long 

 time, if not throughout life, unanchylosed 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 central por- 

 tion of the acetabulum remains unossified, and hence, in the 

 dry skeleton, appears perforated, as in Aves, Ornithoscelida, 

 and Crocodilia. 



The inner tendons of the external oblique muscles are ossi- 

 fied for a considerable distance ; and these ossifications appear 

 b the dry skeleton as bones, which are articulated with the 



* The plaeentaticn of the Toxodontia and Sirenia is unknown. 



