THE ENDOSKELETON: SKULL AND VISCERAL SKELETON in 



4. The chondrocranium.— When the skull of Necturus is soaked in warm 

 soap solution, the membrane bones can easily be lifted off, revealing the chondro- 

 cranium beneath. The chondrocranium (including the olfactory and otic cap- 

 sules) consists in large part of cartilage with a few cartilage bones, which have 

 already been identified. 



Study prepared chondrocrania. The form of the chondrocranium is similar 

 to that illustrated in Figure 32, p. 97. The posterior part of the chondrocranium 

 consists chiefly of the rounded, hollow otic capsules. These are connected dorsally 

 by a narrow arch of cartilage, the synotic tectum, and ventrally by the broader 

 basal plate, formed by the fusion of the two parachordals. The two exoccipital 

 bones are ossified in the basal plate. From the anterior end of each otic cap- 

 sule a slender curved prechordal cartilage extends forward. Between these is 

 a large space, the basicranial fenestra, which in the complete skull is covered 

 above by frontals and parietals and below by the parasphenoid. Near the 

 anterior end in front of the point where in the complete skull the parasphenoid 

 ends, the prechordals are fused across to form the ethmoid plate, already noted. 

 From the ethmoid plate a slender process continues forward on each side, the 

 cornua or horns of the prechordals. In front of the otic capsule is the quadrate 

 bone and quadrate cartilage. The latter sends processes to the otic capsule and 

 the prechordal. In the walls of the otic capsule, opisthotic, prootic, and colu- 

 mella may be identified and their boundaries more clearly distinguished than 

 in the study of the entire skull. The olfactory capsules are so delicate as to 

 be lost in preparing the chondrocranium. 



5. The lower jaw. — The lower jaw consists of a pair of Meckel's cartilages 

 united in front and sheathed for the greater part of their course in membrane 

 bones. The outer surface of each half of the lower jaw consists of the dentary, 

 a membrane bone bearing teeth. The inner surface is formed of two membrane 

 bones, the splenial and the angular. The former is a small bone situated at 

 about the middle of the inner surface, bearing the last group of teeth, consisting 

 of five or six teeth. The angular covers the remainder of the inner surface and 

 passes onto the outer surface at the extreme posterior end of the jaw, below the 

 posterior end of the dentary. The articulating surfaces of the lower jaw are 

 composed of cartilage, which is the posterior end of Meckel's cartilage. This 

 articulates with the quadrate of the upper jaw. Meckel's cartilage runs almost 

 the entire length of the jaw concealed between the dentary and the angular. It 

 can be revealed by removing these membrane bones. 



6. The remaining gill arches.— The hyoid and three succeeding gill arches 

 are present in Necturus and are almost completely cartilaginous. Preserved 

 material is necessary for their study. The hyoid arch is a broad somewhat 

 V-shaped cartilage situated just posterior to the lower jaw in the floor of the 

 mouth cavity. On each side it is divisible into two cartilages— a small anterior 

 hypohyal and a much larger posterior ceratohyal. The third gill arch (first true 



