640 



MAMMALIA. 



parietal, and thus an arcli is formed of sufficient span to cover the 

 middle lobes of the cerebrum. 



The anterior, or frontal vertebra, Fig. 296. 



has for its body the anterior sphenoid 

 (al<E minores) ; its arch being com- 

 pleted by the cavity of the os frontis, 

 which encloses anteriorly the cribriform 

 plate of the ethmoid bone. 



From this analysis of the compo- 

 sition of the cranium, it is apparent 

 that the temporal bones, although in 

 Man they assist so materially in com- 

 pleting the cranial cavity, are only 

 intercalated between the real vertebral 

 elements ; as indeed might almost have 

 been anticipated, seeing how different- 

 ly the pieces belonging to this bone 

 are arranged in different classes of 

 Vertebrata. 



(721.) Such is the general organ- 

 ization of the Mammiferous skeleton. 

 Let us now proceed to consider the 

 osteology of the different orders into 

 which the Mammalia have been dis- 

 tributed, and observe in what respects 

 they individually differ from each 

 other. 



The transition from Birds to Qua- 

 drupeds, remotely separated as they 



might appear to be, is effected by gentle gradations of struc- 

 ture ; and the MONOTREMATA, notwithstanding their quadru- 

 pedal form and hairy covering, are so nearly allied to the feathered 

 Ovipara in many points of their organization, that they evidently 

 form a connecting link between these two great classes of animals. 



It is true that they have mammary glands, and must therefore 

 be supposed to give suck to their offspring ; but it is not even yet 

 satisfactorily determined whether they lay eggs, or produce living 

 young. The structure of their generative apparatus would seem, in 

 fact, to be rather allied to the Oviparous than the Mammiferous 

 type; and, as in Birds, the rectum, the urinary passages, and the 

 sexual organs, all discharge themselves into a common cloacal 



