27] THE SKULL OF AMIURUS— KINDRED . 27 



for the support of the operculum projects from the posterior margin just below 

 its articulation with the otic capsule. 



The opercular bones are present at this stage as thin curved sheets of fibrous 

 connective tissue posterior to and below the hyomandibular cartilage. 



Meckel's cartilage has fused in front with its fellow of the opposite side 

 forming a continuous lower jaw. The anterior part of the cartilage is slender 

 and cylindrical, but posteriorly, just before articulation with the quadrate, 

 there is a sharp process on the dorsal surface and the dorso-ventral diameter 

 is greatly increased. A small piece of the cartilage extends posteriorly beyond 

 the quadrate articular surface. 



In Elasmobranchs (Holocephali excepted) the palatine cartilage is continu- 

 ous with the pterygo-quadrate bar and abuts against the lateral surface of the 

 trabecula by the palatal process. Its anterior extent is not as great as in 

 Amiurus and it never comes in contact with the ectethmoid process, but fuses 

 with the pars palatina of the opposite side forming a complete upper jaw. 

 Since it is supported by the hyomandibula posteriorly and touches the cranium 

 anteriorly, this type of jaw has been termed amphistylic. By dismemberment 

 of the primitive upper jaw of the Elasmobranchs, the condition of Amiurus is 

 brought about, and the palatine part of the arch is separated from the ptery- 

 goquadrate part. Steps in this process occur in the ganoids (Polypterus) and 

 in the teleosts (Salmo). In Polypterus (Budgett, 1900) the palatine is con- 

 tinuous with the pterygoquadrate part, as in the Elasmobranchs, but has an 

 extent much greater anteriorly. There is no palatal process as in the latter 

 group and the anterior end is attached to the ventral surface of the nasal cap- 

 sule. In Salmo (Parker, 1872; Gaupp, 1906) the palatine cartilage is at first 

 independent and secondarily fuses with the pterygoquadrate. It articulates 

 with the ventral surface of the ectethmoid process, and, at a point anterior to 

 this, with the ventral surface of the solum nasi. The part of the palatine 

 which articulates with the ectethmoid process is club-shaped and larger than 

 the more posterior part. Thus there are important differences in the shape 

 of the cartilage and the manner of articulation when compared with Amiurus. 

 The wider separation of the palatine cartilages and their articulation with the 

 lateral surface of the ectethmoid process may be the result of the depression of 

 the cranium. In Gymnarchus (Assheton, 1907) the palatine part of the 

 palatopterygoid cartilage is fused with the ventral surface of the cranium, 

 thus differing radically from the condition in Salmo and Amiurus. 



In all of the forms referred to above, with the exception of Amiurus, the 

 quadrate is a thickened mass of cartilage connected anteriorly with the pars 

 palatina by the pterygoid cartilage and bearing on its ventral posterior part a 

 surface for the articulation of Meckel's cartilage. The quadrate cartilage of 

 Amiurus corresponds in its relations to those of the other forms, but it is fused 

 dorso-posteriorly with the hyomandibula (Fig. 1). In Polypterus, the hyo- 

 mandibular cartilage is dumbbell-shaped, one of its clubbed ends articulating 



