136 MAMMALIAN DESCENT. [Lect. V. 



eiitherian skull ; by it you cau in some degree measure the 

 various kinds and degrees of sj)ecialisation to l)e seen in 

 the other and higlier kinds. In the skull of the embryo 

 Hedirehoof we see several characters that are familiar to us 

 in that of an embryo Reptile or Bird ; and some are like 

 what are seen also in the Marsupials. As we ascend in 

 the scale of the Orders these quasi-reptilian and 

 marsupial (metatherian) characters die out, more or 

 less. 



Beginning with the superficial bones that cover the 

 inner or cartilaginous cranium — Ijesides the flatness of 

 their form, covering as they do, a flat skull with a small 

 internal cavity — we see that the single j^late, called 

 '' interparietal," is very large. This addition to the 

 occipital arch is jDeculiar to certain groups. 



In my former lectures I have frequently spoken of 

 the gradual specialisation of dermal scutes (or j)lates of 

 the superficial armour) as we ascend in the scale of the 

 Vertebrata; the internal skull — with its contained organs 

 — dominating the outer ^^arts, making them answer to 

 what is wanted for protection, both in numl)er, in 

 weight, and in measure. Now, in fish and in reptilian 

 forms, besides the main frontal l)ones, we frequently 

 find a series over the brows, the supra-orl)ital scales ; 

 these linger even in the l)ird class. As yet, in no 

 mammal except this, have I found more than one 

 frontal bone, right and left; but, for a wliile, in the 

 Hedgehog, the orl)ital rim and 2)late are separate, as 



