CRANIAL OSTEOLOGY OE THE PHRACTOL^MIDai. 279 



The interopercular is remarkably large, o£ about twice the size 

 of the opercular bone ; it extends well forward, its poiuted 

 anterior extremity nearly reaches the front of the head, and there 

 is a considerable overlapping o£ the right and left interopercular 

 bones beneath the head. Along the upper edge of the inter- 

 opercular, and concealed by the overlapping lower edge of the 

 suborbital plate, is a branching sensory canal received from the 

 lower end of the preopercular. The interopercular is thus here 

 performing the function of the missing horizontal limb of the 

 preopercular. This is the only instance known to me in which 

 the sensory canal passes through the interopercular bone : even 

 in Lepidosteus, in which the interopercular is situated at the 

 front of the preopercular, the canal does not pass across the 

 interval between the preopercular and the back of the mandible 

 through the interopercular bone, but through the skin (see 

 CoUinge, Proc. Birm. Phil. Soc. viii. 2, 1893, p. 265 and pi. 8). 



Although the mouth-parts are extremely specialised, the 

 bounding of the gape above is effected, as in the less specialised 

 fishes generally, by the premaxillaries mesially and by the max- 

 illaries laterally. The mouth is very remarkable in form, and is 

 described by Boulenger (I. c. jd. 6) as " small, proboscidiform, 

 ■capable of being thrust forward, when at rest folded over and 

 received into a depression on the upper surface of the head." 

 This depression faces upwards, and is bounded in front by the 

 interior suborbitals and behind by the mesethmoid. When 

 the mouth is withdrawn, the maxillae lie against the front of 

 the mesethmoid, and rest on the ledge formed by the front of the 

 vomer ; but when the mouth is protruded, a large tract of skin 

 intervenes between the maxillae and the mesethmoid. 



The mouth-skeleton, when the mouth is extruded, is almost 

 detached from the other parts of the skull. It forms a ring of 

 bones around the mouth-opening. Each maxilla is slender, 

 curved into an irregular semicircle, the lateral (^. e. the morpho- 

 logically posterior) extremity being expanded and attached by 

 fibrous tissue to the outer surface of the upper extremity of the 

 •dentary. The two maxillae nearly meet in the median plane of 

 the head, and ai'e connected with one another by fibrous tissue. 

 The two premaxillae are set immediately in front of the mesial 

 ends of the maxillae; they have the form of flat, triangular plates 

 -of bone, and, being small, they support only the upper part of the 



