114 UN THE MORPHOLOGICAL CONSTITUTION OF 



Keeping out of view, therefore, the cyclostomatous fishes 

 and the proboscidian mammals, which present indications of 

 a greater number, the vertebrata generally possess all the 

 sclerotomes enumerated above, except the rhinal, which 

 exists only in mammals and crocodiles. 



On a Fundamental Difference between the Cranium of the 

 Mammal and that of the Bird, Reptile, Amphibian, and 

 Osseous Fish. In my earlier attempts to unravel the intri- 

 cacies of this subject, I found myself opposed by difficulties 

 in passing from the mammalian to the lower form of cranium, 

 and vice versa. I afterwards discovered that this mainly de- 

 pended on the reciprocal development and atrophy of the 

 meta-neurapophyseal elements of four sclerotomes in the two 

 forms. In consequence of this, we had been hitherto con- 

 founding the frontal bone, or meta-neurapophysis of the eth- 

 moidal sclerotome of the mammal, with the so-called "proper 

 frontal bone," which is in fact the meta-neurapophysis of the 

 pre-sphenoidal sclerotome of the bird, reptile, amphibian, and 

 osseous fish, an element of which there are, and this only in 

 rare instances, faint or doubtful traces in the former; and 

 pari passu, we had been confounding the parietal bones, the 

 double meta-neurapophysis of the post-sphenoidal sclerotome 

 of the mammal, with the so-called parietals, the largely-de- 

 veloped meta-neurapophyses of the temporal sclerotome of 

 the bird, reptile, amphibian, and osseous fish, elements which 

 are much reduced in size, and masked in the former. Among 

 other important organic relations indicated in the existence 

 of these two forms of cranium, I would here more particularly 

 note their bearing on the encephalon. Of the two forms, that 

 of the fish, reptile, and bird, while it adheres to the common 

 type, is modified mainly in relation to the organs of smell, 

 sight, and hearing. That of the mammal, also adhering to 

 the common type, is modified in relation to the cerebrum 

 proper to that nervous structure superimposed upon the 



