136 MAMMALIAN DESCENT. [Lect. V. 



eutherian skull ; hj it you can in some degree measure the 

 various kinds and degrees of specialisation to be seen in 

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

 Hedgehog 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 cjuasi-reptilian and 

 marsupial (metatherian) characters die out, more or 

 less. 



Beginning with the superficial bones that cover the 

 inner or cartilaginous cranium — besides the flatness of 

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

 internal cavity — we see that the single jjlate, called 

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

 occipital arch is peculiar to certain groups. 



In my former lectures I have frecpiently sjioken of 

 the gradual specialisation of dermal scutes (or plates of 

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

 Yerteljrata ; the internal skull — with its contained org^ans 

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

 what is wanted for protection, both in numljer, in 

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

 forms, besides the main frontal bones, we frecjuently 

 find a series over the brows, the supra-orbital scales'; 

 these linger even in the bird class. As yet, in no 

 mammal except this, have I found more than one 

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

 Hedgehog, the orljital rim and jDlate are separate, as 



