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groove. The bone is traversed its füll length by tbe preopercular latero-sensory canal, and lodges 

 six sensory organs. 



The HYOMANDIBULAR is an irregulär bone, roughly triangulär in general outline, one of 

 the angles of the triangle being directed dorsally and bearing the articular heads of the bone. Not 

 far from the anterior edge of the triangle, and parallel with it, there is a slightly thiekened portion, 

 which represents the shank of the bone. The hind edge of the triangle is slightly convex, and bears 

 on its external surface a tall ridge which projects laterally and slightly anteriorly. The posterior 

 surface of this ridge is applied against, and firmly bound to the antero-mesially presented anterior 

 surface of the dorsal lirnb of the preopercular, the dorsal end of the latter bone reaching to the lower 

 edge of the opercular process of the hyomandibular. This tall ridge along the hind edge of the hyo- 

 mandibular of Dactylopterus thus corresponds to the ridge that extends longitudinally along the 

 external surface of the shank of the bone in the other fishes of the group; but here, in Dactylopterus, 

 although the ridge begins on the external surface of the dorsal end of the slightly indicated shank, 

 it extends postero-ventrally at an angle to the shank, and leaves, between itself and the shank, a 

 wide intervening space which is spanned by a thin web of bone. The dorsal end of the ridge is 

 thiekened to form a large articular head, which articulates with the facet on the antero-lateral corner 

 of the squamosal, and is accordingly the regulär posterior articular head of the bone. Postero-mesial 

 to this articular head there is a stout and bluntly pointed process, directed antero-dorsally in the line 

 of the ridge on the external surface of the bone. The postero-lateral surface of this process is slightly 

 coneave, is grooved from its top to its base, and has a sliding articulation with the articular surface on 

 the thiekened posterior edge of the articular facet on the squamosal. From the anterior surface of the 

 base of this articular process a strong process is directed antero-ventro-mesially. It expands at its 

 distal end, and has a curved articular edge which is presented mesially and has a sliding articulation 

 with the articular surface on the postorbital edge of the proötic immediately dorsal to the trigemino- 

 facialis charnber. Fused with the lateral edge of the base of this process of the hyomandibular, there 

 is a small process which arises from the anterior surface of the ridge on the external surface of the 

 bone, immediately ventral to the regulär posterior articular head. This small process is directed 

 anteriorly and has on its dorsal surface a slightly coneave articular surface which articulates with the 

 articular eminence on the sphenotic. This small process accordingly represents the regulär anterior 

 articular head of the bone. Each of the two regulär articular heads of the hyomandibular of Dactylo- 

 pterus thus has a related, accessory articular head which has a sliding articulation with the cranium, 

 this arrangement giving great solidity to the Joint. 



On the hind edge of the bone there is a short opercular process. 



On the internal surface of the bone, immediately ventral to the bases of the two accessory 

 articular processes, there is a slight transverse ridge of bone, and on the dorsal surface of this ridge 

 is the internal opening of the facialis canal. This canal traverses the hyomandibular and opens on 

 its external surface immediately posterior to the slightly thiekened portion that represents the shank 

 of the bone. Immediately ventral to the transverse ridge, on the internal surface of the bone, a large 

 opening leads directly into the facialis canal, and this opening transmits the ramus hyoideus, this 

 nerve thus never reaching the external surface of the hyomandibular. The remainder of the facialis 

 canal transmits the ramus mandibularis lateralis certainly aecompanied by communis fibers, and 

 possibly also by certain general cutaneous fibers. The nerve so constituted is joined, as it reaches 

 the outer surface of the bone, by a eommunicating general cutaneous bündle which arises from the 



