126 ON THE MORPHOLOGICAL CONSTITUTION OF 



if it appears in the fish at aU, it is a cartilaginous or seraiossi- 

 fied nodule between the intermaxillaries. That the centrum 

 of the ethmoidal sclerotome in the fisli, considered as the 

 homologue of the central plate or bar of the mammalian 

 ethmoid, should carry teeth in the fish, is not more remarkable 

 than that one of the centrums of the cervical vertebrae in that 

 class of animals should be so armed. 



Hcemal Arch of the Ethmoidal Sclerotome in the Mammal 

 and Osseous Fish. — I have commenced my account of the 

 morphological constitution of this important sclerotome by 

 pointing out the typical arrangement which its neural arch 

 and centrum present in the mammal and fish. As the arrange- 

 ment of these parts of this sclerotome becomes much and 

 variously modified in birds, reptiles, and amphibia, in relation 

 to the various forms presented by the organs of smell and the 

 nostrils, it will be necessary, before proceeding further, to 

 examine the constitution of its htemal arch. Even in its most 

 complex form, this hsemal arch, like those of the rhinal and 

 vomerine sclerotomes, consists of two elements only, the right 

 and left maxillary bones. In the osseous fish they resemble 

 the " lateral frontal processes " in the embryo ; they form only 

 an incipient arch like that formed by the vomerine hsemapo- 

 physes in front of them. They do not invariably carry teeth. 

 They are variously connected to the haemapophyses before 

 and behind them, and superiorly to the lateral and fore part of 

 the centrum, neurapophyses, and metaneurapophyses of the 

 neural arch of their own sclerotome. 



The maxillaries of the mammal, more or less extended 

 from before backwards, and increased in breadth and depth to 

 adapt them to their functions in mastication ; meeting one 

 another below, to form a great part of the vault of the palate, 

 and to assist in the formation of the nasal passages ; hollowed 

 out to combine lightness with strength ; and buttressed by 

 numerous connections with neighbouring bones, nevertheless 



