i8 LABORATORY DIRECTIONS 



of the Necturus skull, namely, exoccipital, prootic, opisthotic, 

 and quadrate, are best seen In the chondrocranium preparations. 



The lower jaw consists of membrane bones surrounding 

 Meckel's cartilage. The dentary bone, bearing teeth, constitutes 

 the outer surface of each half of the jaw. The inner surface is 

 composed of two bones, the splenial and the angular, the latter 

 forming the extreme posterior end of jaw below the dentary. In 

 Cryptobranchus ossification of a portion of Meckel's cartilage 

 to form the articulating surfaces of the jaws has begun. This 

 bone is the articular; it articulates with the quadrate bone of 

 the upper jaw. 



Examine the visceral arches on a mounted skeleton of 

 Necturus. The hyoid arch is the bar of cartilage parallel and 

 posterior to the lower jaw. The three gill arches are remnants 

 of the third, fourth and fifth visceral arches. As compared to 

 the shark the gill arches are not only reduced in number but 

 also in the cartilages composing them. 



Skull of the Alligator 



The skull of the alligator is almost completely ossified, the 

 chondrocanium having been replaced by bone. Although the 

 alligator skull will not be studied in detail, the following features 

 should be noted: (i) There are more bones present than in 

 mammal skulls. (2) There is but one occipital condyle. (3) 

 There are no teeth on the roof of the mouth as in Necturus, 

 Cryptobranchus, and many fishes; teeth are borne on the 

 maxillary, pre-maxillary, and dentary bones only. (4) The 

 quadrate bone, an ossified remnant of the pterygo-quadrate 

 cartilage, forms the articulation with the lower jaw. (5) The 

 lower jaw consists of two halves or rami united in front by a 

 symphysis. The bones of each half are: the dentary, bearing 

 teeth and forming the outer two-thirds of each jaw; the splenial, 

 forming a large part of the inner side; the angular, below the 

 large mandibular foramen; the sur-angular above the foramen; 

 the coronoid, a small bone on the Inner surface forming part of 

 the anterior boundary of the mandibular foramen; and the 

 articular, which contains a concavity for articulation with the 

 quadrate of the upper jaw. The articular Is an ossified portion 

 of Meckel's cartilage. A remnant of Meckel's cartilage still 



