94 ILLINOIS BIOLOGICAL MONOGRAPHS [94 



which it had in the younger stages. It tapers anteriorly where it meets the 

 fellow of the opposite side and is deeper posteriorly (Fig. 15). The inner sur- 

 face of the bone is hollow for the reception of the anterior end of Meckel's 

 cartilage. The ventro-anterior surface is perforated by a series of six pores 

 for the passage of tubules of the enclosed mandibular lateral line canal (Fig. 

 11). The canal issues from the mandible through the more posterior pore 

 and enters the connective tissue surrounding the quadrate and thence passes 

 into the preopercular. I agree with McMurrich that the dentary is a mixed 

 bone and is the result in part of the ossification of a portion of Meckel's carti- 

 lage. The developmental stages show that the bone is at first arises entirely 

 from the connective tissue membrane aroimd the cartUage fused with a lateral 

 line ossification; later part of Meckel's cartilage ossifies and fuses with it 

 (Fig. 24). 



The articulare. Tliis is a triangular bone at the posterior end of the mandi- 

 ble and is grooved on its posterior face for articulation with the quadrate 

 (Fig. 15). It does not contain a lateral line element. The internal dorsal 

 surface is flat and gives attachment to the muscles which move the lower jaw. 

 Considerable cartilage persists between the outer and inner lamellae. The 

 bone bears no teeth and interdigitates anteriorly mth the dentary, which also 

 extends posteriorly in a groove on its ventral surface. 



The preopercular. The mandibular-opercular lateral :ine canal entersth is 

 bone from the connective tissue surrounding the posterior ventral end of the 

 quadrate (Figs. 11, 15). The bone is fused solidly to the quadrate ventrally, 

 and to the hyomandibular dorsally. A small process projects down behind 

 the quadrate and the opercular canal passes into this part. 



The posterior edge is ridged and projects above the more posterior lying 

 operculum. The dorsal end of the bone extends for a short distance behind 

 the hyomandibular. There is a small foramen in the middle part of the bone 

 for passage of a branch of the hyomandibularis facialis as it descends after 

 passing through the hyomandibular. 



The suUemporals. These are two small bones lying dorsal to the preoper- 

 cular and above the posterior margin of the hyomandibular (Figs. 11, 15). 

 They contain the dorsal end of the opercular lateral line canal which passes 

 through them into the squamosal. They are very thin and the canal in pass- 

 ing through them, hes in a groove rather than in a tube, the ventral and lateral 

 walls of which are thicker than the very thin roof. Only one such bone, having 

 the same relations to the lateral line canal and the squamosal is found in this 

 region of Salm^o. Parker (1872) called it the supratemporal, but the supra- 

 temporal of the stegocephalans always lies medial to the squamosal so that this 

 bone could not be its homologue. Ridewood (1904) suggests the term adopted 

 here and says that the real supratemporal of Salmo, between the back of the 

 squamosal and the post-temporal, was overlooked by Parker, but it is really 



