ALIMENTARY CANAL. 63 



sphenoid region. It is indeed fused all along with the side of 

 the cranium and has the form of a laterally projecting tri- 

 angular shelf, the projecting angle of which is the quadrate 

 region and gives articulation to Meckel's cartilage. This 

 arrangement is called Autostylic ; * the hyoid arch taking no 

 part in the suspension of the upper jaw.f It has been proposed 

 to divide fishesi nto two great groups based upon the condition 

 of the primitive upper jaw skeleton the Autostylici and the 

 Hyostylici ; but as we shall explain in the sequel, there appear to 

 be good reasons for adopting a different arrangement. 



In the Teleostei, Ganoidei and Dipnoi cartilage bones are 

 developed in both the mandibular and hyoid arches to a varying 

 extent and membrane bones may come to overlie them, largely 

 supplementing them and even replacing them. 



The digestive organs vary much in structure. The mouth, 

 which is placed at, or near, the anterior end of the head, usually 

 has the form of a transverse slit, and can sometimes be extended 

 forward by means of the movable supporting bones of the upper 

 and lower jaws. The buccal cavity is distinguished by its width, 

 and by the great number of teeth it contains, which are developed 

 from the papillae of the mucous membrane by dentinal ossifica- 

 tion. There are often two curved parallel rows of teeth on the 

 upper jaw ; an outer row on the premaxilla, and an inner row 

 on the palatine, and there may also be a median unpaired row on 

 the vomer. On the lower jaw there is only one curved row of 

 teeth. There may also be teeth on the* hyoid arch and on the 

 maxillae, pterygoids, and parasphenoid, and, as a rule, on the 

 branchial arches also, especially on the upper and lower pharyn- 

 geal bones. The teeth may be distinguished according to their 

 shape into pointed conical prehensile teeth, and grinding teeth. 

 They are developed in the mucous membrane and are attached to 

 the skeletal structures by ligament or by ankylosis. In a few 

 cases only are they implanted in sockets. . 



A small, hardly movable tongue is developed on the floor of the 



* The so-called amphistylic arrangement which is found in a few Elasmo- 

 branchs (see below) would seem to be a variety of the autostylic. 



t The suspension of the mandibular arch, found in the skulls of Am- 

 phibia and Sauropsida, in which the palato-quadrate bar is not attached 

 along its whole length, but only in the auditory and ethmoid region, 

 must be regarded as a more typical form of the autostylic arrangement 

 than that found in Chimaera and the Dipnoi . 



