THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 101 



of vertebrates the chondrocranium consists chiefly of a ventrally located plate and lacks side 

 walls and roof. Walls and roof are completed by these dermal scales, which thereupon 

 become the superficial bones of the skull. Similarly dermal scales incase the pterygoquadrate 

 and Meckelian cartilages, which are the primitive upper and lower jaws, and become the super- 

 ficial bones of the jaws. In this fashion the entire skull becomes sheathed in dermal scales, 

 originating in the skin and in reality belonging to the exoskeleton. These incasing bones of 

 the skull and jaws are known as dermal, membrane, or investing bones, because of their 

 manner of origin. The membrane bones of the skull are present in their most complete and 

 generalized condition in the earliest Amphibia (Stegocephala) and the earliest reptiles (Coty- 

 losauria) ; but in the course of evolution gradually decrease in number owing to processes of 

 loss and fusion (see Fig. 36, p. 106). 



2. Membrane bones of some typical ganoids. — Study the ganoid scales on 

 the head of any one of the following ganoids: Polypterus, sturgeon (Acipenser), 

 the gar pike (Lepidosteus) , or the bowfin {Amid). Some of these enlarged 

 scales on the heads of ganoid fishes correspond to the superficial bones of the 

 skull of higher vertebrates, and have received the same names; others, especially 

 those supporting the operculum, have been lost subsequently. The student 

 should understand that these dermal scales are on the outside of the animal's 

 head, imbedded in the skin, and that a typical cartilaginous or partially ossified 

 chondrocranium, like that of the dogfish, is present inside of the covering of 

 scales. 



a) Membrane bones of Polypterus: (see Fig. 34^) The dermal scales on the 

 head of Polypterus are very much like the bones on the dorsal side of the 

 skull of higher forms. Identify on the dorsal side of the head the two small 

 nasal openings near the tip and the larger oval orbits posterior to them. In 

 front of the nasal openings are a pair of bones bearing teeth, the premaxillae; 

 behind the nasal openings, a pair of nasals; between the two nasals, a portion 

 of the chondrocranium called the mesethmoid reaches the surface. Posterior 

 to the nasals are two large frontal bones, and posterior to them, two smaller 

 parietal bones. The several small bones posterior to the parietals are called 

 temporals. A row of small bones extends from each orbit posteriorly; these are 

 postorbitals. Below the orbit is an elongated bone bearing teeth, the maxilla. 

 The sides of the head behind the orbit are covered by large flat bones, the oper- 

 culars. The lower jaw is similarly clothed in dermal bones, consisting of a 

 tooth-bearing dentary in front and a toothless angular behind. 



b) Membrane bones of the sturgeon: (Fig. 34C) The anterior end of the 

 sturgeon's head is extended into a snout or rostrum, covered by many small 

 rostral scales. At the sides of the base of the snout are the two pairs of nasal 

 openings, and just posterior to them, the orbit. On the dorsal side between 

 the two orbits are two large scales, the frontals, and posterior to them, two 

 parietals. Numerous other small bones need not be considered. The jaws and 

 visceral skeleton of the sturgeon are degenerate on account of its method of 

 feeding. 



