26 ILLINOIS BIOLOGICAL MONOGRAPHS [26 



tion in Salmo is lacking in Amiurus, but that which extends medially beneath 

 the cranium corresponds in the two forms. I find no trace of a labial cartilage 

 in Amiurus. In Salmo this lies between the cartilage of the ethmoid plate 

 and the premaxillary ossification. 



In the 10 mm. Amiurus the maxillary bone is represented by a small ossifi- 

 cation lateral to the anterior end of the palatine cartilage (Fig. 30). The 

 medial surface of this piece curves half way around the palatine cartilage and 

 a small process projects laterally from its external surface, above the proximal 

 end of the maxillary barbel. There are no teeth connected with the ossifica- 

 tion. In Salmo, (Schleip, 1903) the maxillary ossification abuts against the 

 palatal cartilage in the same region as in Amiurus, but teeth arising below it 

 fuse with it later. 



The palatine cartilage is a slender cylindrical bar lying lateral to and parallel 

 with the ethmoid plate (Figs. 1, 2). It is not uniform in diameter, but tapers 

 posteriorly. Its anterior end lies lateral to the nasal organ (Fig. 30) and pos- 

 terior to it the medial surface is flattened for articulation with the lateral sur- 

 face of the ectethmoid process. The posterior end of the palatine cartilage 

 is connected with the anterior end of the pterygoid cartilage by a thin stroma 

 of connective tissue cells, among which I was unable to find any cartilage cells. 

 Muscle fibres extend to the dorsal and ventral surfaces of the posterior end of 

 the cartilage from the margin of the ethmoid plate ventral to the orbital fora- 

 men. 



The pterygoquadrate cartilage consists of an anterior, slender, flat ptery- 

 goid bar and a posterior thickened quadrate cartilage (Fig. 1). The ptery- 

 goid part extends anteriorly beneath the eye, parallel with the trabecula cranii 

 and connected to it by a sheet of muscle fibres. It ends there and is connected 

 with the posterior end of the palatine cartilage by the stroma of cells, men- 

 tioned above. Its posterior end descends obliquely toward the lower jaw and 

 passes into the quadrate cartilage just dorsal to its articulation with Meckel's 

 cartilage. The ventral anterior surface of the thickened quadrate cartilage 

 is concave where it articulates with the dorsal surface of Meckel's cartilage; 

 postero-dorsally the quadrate is indistinguishably fused with the hyomandibu- 

 lar cartilage. 



The anterior end of the dorsal margin of the hyomandibular cartilage abuts 

 against the latero- ventral surface of the posterior end of the ahsphenoid carti- 

 lage (Fig. 1). From this point the articular surface for the hyomandibula 

 extends obhquely dorso-posteriorly along the lateral wall of the otic capsule to 

 a point external to the lateral semicircular canal. From this articulation the 

 hyomandibular descends, as a thin vertical plate of cartilage, from the wall of 

 the otic capsule behind the quadrate and is connected at its ventro-posterior 

 end with hyoid cartilage by the interhyal. Its anterior ventral margin encloses 

 the hyomandibularis branch of the faciahs nerve. A cartilaginous process 



