STRUCTURE OF THE VERTEBRATES 131 



Jaws. In the bony fish, amphibia, reptiles and birds the 

 pterygo-quadrate cartilage ossifies as the pterygoid and the 

 quadrate bones. The latter is the ossified posterior end and there- 

 fore articulates with the lower jaw. The posterior end of Meckel's 

 cartilage ossifies as the articidar bone, being in contact with the 

 quadrate. The anterior portion of this cartilage disappears with 

 the growth and development of the dermal covering. 



In the primitive reptiles there were seven bones on either side 

 of the lower jaw, one replacement bone (articular) and six dermal 

 bones. A comparative study shows that the dentary, the most 

 anterior of the dermal bones, pushed farther back during the 

 course of evolution, occupying more and more of the jaw. In the 

 Theromorpha (page 59) the posterior elements, including the 

 articular, are small; and in the Therapsida the dentary makes 

 up practically all of the jaw, with the posterior bones a nipple- 

 like structure at the end. The point of articulation is, however, 

 between the quadrate and the articular. In one species of the 

 therapsids there is a secondary point of articulation between the 

 dentary and the squamosal, a dermal bone of the skull. The 

 mammals have completely lost the old articulation. The lower 

 jaw is composed of one bone, the dentary, and this articulates 

 directly with the squamosal. 



Dermal Bones of the Skidl. As one would expect from their 

 origin from dermal denticles and dermal plates, the covering 

 bones of the skull have proceeded from the complex to the simple. 

 The reverse has been true in the more fundamental systems. 

 Therefore, clarity may be gained by describing the dermal bones 

 of a simple mammal first, and comparing afterward. The opos- 

 sum, a marsupial (page 78) is sufficiently generalized to serve 

 the purpose. 



The top of the skull, is covered by four pairs of bones, the 

 longitudinal suture, or point of contact between two bones, being 

 the mid-dorsal line. From posterior to anterior these are: (1) 

 the dermal supraoccipital, completely fused with the replacement 

 occipital and indistinguishable from it except in the embryo; 

 (2) the parietals, covering the posterior part of the brain case 

 and extending over the sides of the skull; (3) the frontals, the 

 longest of the dorsal bones, bordering and extending into the 



