LECT. VII.] A COMPLETE OEBITAL RING. 173 



well as on the main land, even as far as to India, there 

 is a kind of Insectivore the Tupaia, which is very 

 isolated from the others in the Order. The species of 

 this genus are elegant squirrel-like creatures, but they 

 are rather related to the Lemurs than to the Eodents. I 

 only know this type in the adult state, and I have only 

 carefully studied the skull ; but this is enough to make 

 me long for a fuller acquaintance with it. There are 

 two important things in which this skull differs from 

 that of the Insectivora generally, namely, its complete 

 orbital ring, and its large bulla tympani or additional 

 bony shell to the drum-cavity. The orbital ring is 

 complete in Lemurs, the Primates including Man, and 

 in some Ungulates ; but this is the lowest type of Mam- 

 mal, I believe, in which this character is perfected. In 

 the Tupaia, however, this ring is peculiar, for there is an 

 open space where the zygomatic process of the squama 

 temporis (temporal bone) rests upon the malar or cheek 

 bone. 



This type has, certainly, the most perfect and Cat-like 

 bulla tympani ; yet I suspect that in this case, as in a 

 type soon to be mentioned Rhyncliocyon it does not 

 become cartilaginous before it is ossified : in the Cat 

 this innermost ring of the meatus exists as hyaline 

 cartilage for some time. This isolated group of Insecti- 

 vora the Tupaiaidse comes in with excellent evidence 

 as to the origin of the higher forms of the Eutheria : all 

 these mixed or generalised types are the living, but 



