— 69 — 



slightly dorsal to it, on the raised and ridge-like cxternal surface of the preopercular, there 

 is a shallow groove which marks the line of insertion of the hind end of the third bone of the 

 infraorbital ehain. 



OPERCULAR BONES. 



The three opercular bones have the shapes shown in the figures. On the external surface of 

 the opercular, three pronounced ridges radiate from the articular facet of the bone. One of these 

 ridges forms the ventral edge of the bone, which edge is presented anteriorly and but slightly vent- 

 rally; the other two ridges lying on the dorsal portion of the bone and both of them terminating 

 in free spines. The internal surface of the bone, dorsal to the dorsal one of the three spinous ridges, 

 is depressed, and in this depression the adductor operculi has its insertion; the thin and almost mem- 

 branous levator operculi being inserted along the dorsal edge of the bone. The deep indentation in 

 the hind edge of the bone, between the two dorsal spines, corresponds to the indentation, without 

 related spines, in the hind edge of the bone of Scomber. 



The angular-shaped subopercular overlaps internally, and embraces the ventral corner of 

 the opercular, extending upward one half to two-thirds the length of its anterior edge, but along 

 its entire posterior edge; usuallv projecting upward slightly beyond the dorsal edge of the bone. 

 It is a thin, flat bone, its long dorso-posterior arm, in particular, being so thin that it is flexible and 

 easily torn. 



The dorso-posterior edge of the interoperculum is slightly concave, and lies in a nearly horizontal 

 position. The posterior corner of this edge slightly overlaps externally, and is strongly bound by tissue 

 to the ventral corner of the subopercular, while the anterior corner lies external to the interhyal, 

 and is strongly bound to it by tissue. The lateral surface of the ventral half of the interhyal here 

 fits into a large but shallow depression on the internal surface of the interopercular, this depression 

 having a raised dorsal edge which gives it the appearance of an articular facet. The interopercular 

 is thus here related to the interhyal somewhat as the branchiostegal rays are to the ceratohyal, 

 suggesting it being such a ray. Between its concave postero-dorsal edge and the anterior edge of the 

 subopercular there is a large triangulär space, spanned by a sheet of tough fibrous tissue which 

 connects the bones. The ventral end of the interopercular is directed antero-ventrally and gives 

 attachment to a short strong ligament which has its origin on the hind end of the angular. 



In Phractolaemus Ansorgii. Ridewood says ('05 a, p. 279) that the interopercular is traversed 

 by a portion of the preopercular latero-sensory canal, adding that this is the only instance of the kind 

 known to him. If the section of canal enclosed in the bone lodges a latero-sensory organ, the bone 

 can not be a simple interopercular. 



M A N D I B L E. 



The mandible has, on its outer surface, a large rounded longitudinal ridge which extends 

 from the ventral edge of the articular facet for the quadrate forward across the articular and then 

 across the dentary, nearly to the anterior end of the latter bone. On the inner surface of the artic- 

 ular and dentary there is a corresponding hollow, which lodges, in its ventral portion, the rod-like 

 Meckel's cartilage. Immediately ventral to the ridge, the mandibular latero-sensory "canal traverses 

 the dentary and articular, entering the dentary near its anterior end and leaving the articular at 

 the base of the process that forms the posterior half of the articular facet for the quadrate. The 



