Nos. IAND2.] ANATOMY OF SCOMBER SCOMBER. 1,5 



dorsal end of the bone reaching- to, or sHghtly beyond, the dorsal 

 end of the groove. The ventral portion of its anterior edge lies 

 in a groove on the ventro-posterior edge of the quadrate, the 

 lower end of the preoperculum reaching almost to the base of the 

 articular head of that bone. The middle portion of its anterior 

 edge lies external to the hyomandibulo-sym'plectic, and there is 

 here, on the internal surface of the edge of the preoperculum, a 

 relatively deep depression. It lodges the interspace of cartilage 

 that lies between the hyomandibular and symplectic, the adjoining 

 ends of those two bones, the articulating end of the epihyal, and 

 the dorsal end of the dorso-posterior process of the quadrate. 



The preoperculum is firmly bound by fibrous tissue to the hyo- 

 mandibular and quadrate and to the hyomandibulo-symplectic 

 interspace of cartilage, but it is not so bound to the symplectic, 

 a space being left between it and the latter bone for the passage 

 of the mandibularis externus facialis. 



The preoperculum is traversed its full length by the preopercular 

 lateral canal. 



The Operculum (OP) is a somewhat fan-shaped bone, the 

 handle of the fan being thickened, and having, on its end, a round 

 articular facet which articulates with the opercular process of the 

 hyomandibular. This facet, like the corresponding facet in Auiia, 

 is not lined with cartilage, and no in4ication of cartilage could 

 be found in any other part of the bone. Scomber and Aniia thus 

 both dififer in this respect from the condition described by Gegen- 

 baur (No. 29) in Alepocephalus. 



Immediately external, that is, lateral, to the edge of the articular 

 facet of the operculum there is, in Scomber, a strong process, 

 which gives insertion to the tendon of the dilatator operculi mus- 

 cle. The summit of this process lies approximately on a level 

 with, and a little posterior to, the dorsal end of the preoperculum. 

 On the internal surface of the operculum, dorso-posterior to the 

 articular facet, there is a deep depression which forms the surface 

 of insertion for the levator operculi. In the posterior edge of the 

 bone, dorsal to its middle point, there is an indentation of variable 

 form, and across it a layer of degenerate muscle fibers extend. The 

 indentation thus seems to represent the remnant of a space that 

 fomerly existed here between two branchiostegal rays that have 

 fused to form a part, or all, of the operculum. The ventral edge 



