THE SKELETON 33 



ml, pi). Finally, in the bunodont series, the addition of a postero- 

 internal cusp (Fig. 5, A?/), termed the hypocone, forms the sextuber- 

 cular molar. 



The following table exhibits, in a collective form, the names 

 and relations of all the above-mentioned cusps, and the letters by 

 which they are indicated in the figures : 



UPPER MOLARS. 



Antero-internal cusp = protocone =pr. 



Postero or 6th cusp = hypocone = hy. 



Antero-external cusp =paracone =pa. 



Postero ,, =metacone =me. 



Anterior intermediate cusp = protoconule = ml. 



Posterior =metaconule =pl. 



LOWER MOLARS. 



Antero-external cusp = protoconicl =prd. 



Postero =hypoconid =hyd. 



Antero-internal or 5th cusp =paraconid =pad. 



Intermediate (or in quadritubercular 



molars antero-internal) cusp. = metaconid = med. 



Postero-internal cusp = entaconid = end. 



The common occurrence of trituberculism in the mammals of 

 the earlier geological epochs is, as remarked by Osborn, very 

 significant of the uniformity of molar origin. Thus, among the 

 Mesozoic mammals (with the exception of the group known as 

 Multituberculata, in which the molars are constructed on a different 

 type), trituberculism occurs in the great majority of the genera; 

 while out of 82 species, belonging to five different suborders from 

 the Lowest or Puerco Eocene of the United States, all but four 

 exhibit this feature ; and the same holds good for the mammals of 

 the corresponding European horizon. At the present day trituber- 

 culism persists in the Lemuroidea, Insectivora, Carnivora, and Mar- 

 supialia. In the Carnivora there is a tendency to lose the meta- 

 conid, while in the bunodont molars of the Ungulata it is the 

 paraconid that disappears. 



III. THE SKELETON. 



Definition. The skeleton is a system of hard parts, forming a 

 framework Avhich supports and protects the softer organs and 

 tissues of the body. It consists of dense fibrous and cartilaginous 

 tissues, portions of which remain through life in this state, but the 

 greater part is transformed during the growth of the animal into 

 bone or osseous tissue. This is characterised by a peculiar 



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