THE CRANIAL NEEVES AND LATERAL SENSE ORGANS OF FISHES. 125 



that as the sub-orbitals become narrower, so the edge turned over to form the tube 

 approximates more and more to the ventral or posterior edge, the tendency thus being 

 for the whole of the sub-orbital ossicle to be nsed up in the formation of the protective 

 sensory tube. The eighth dermal tubule opens at the dorsal or posterior extremity of 

 the ossicle, the latter tapering off to receive it. 



Fifth Sub-orbital or First Post-orbited. — The iufra-orbital canal has by this time taken 

 a sharp upward curve, so that the fifth sub-orbital is not far removed from the 

 perpendicular. Its length is 17 mm., and the anterior edge almost approximates to the 

 posterior in the formation of the tube lor the iufra-orbital canal. The ninth dermal 

 tubule is received by the infra-orbital canal betweca the fifth and sixth sub-orbitals. 

 This tubule is not rej)resented in Gaclus virens. 



Sixth Sub-orbital or Second Fost-orbital. — This, the last and most posterior sub-orbital, 

 articulates ventrally with the fifth sub-orbital, and dorsally with the postero-lateral region 

 of the frontal, and also slightly with the post-frontal or sphenotic. Its direction is as 

 near as possible perpendicular, and the portion of the infra-orbital canal that it lodges is 

 of course directly continuous with that on the frontal. The length of the canal on the 

 sixth sub-orbital is 14 mm., and it is protected in a precisely similar manner as on the 

 fifth sub-orbital — the anterior and posterior edges not quite approximating. 



Frontal. — The infra-orbital canal is now on the frontal. Situated on the upper surface 

 of the postero-external angle of the frontal is a triangular fossa with the apex directed 

 inwards and forwards. The base of the triangle forms a portion of the posterior edge of 

 the frontal. The infra-orbital canal comes first along the outer side of the ti-iangle (a 

 distance of about 14 mm.), and arriving at the apex first of all anastomoses with the 

 supra-orbital canal, and then turns sharply backwards, edging the inner side of the 

 triangle (about 15 mm.). The canal, therefore, at this region turns first inwards and 

 forwards, and then outwards and backwards. The two sides of the triangular fossa on 

 the frontal are depressed so as to form slight furrows for the reception of the base of the 

 infra-orbital canal. Further, the inner side of the triangle is raised up and arches 

 outwards, thus forming a roof for this portion of the infra-orljital canal. As, however, 

 elsewhere, the tube is imperfect externally. 



Post-frontal or Sphenotic. — The sphenotic now comes to the surface and lodges the 

 next 5 mm. of the infra-orbital canal, the latter stUl pursuing the downward and 

 backward direction of the portion immediately preceding it on the frontal. The 

 sphenotic, however, only supports the floor of the infra-orbital canal, an inner wall and 

 roof being here formed partly by a backward projection of the frontal and partly by a 

 forward prolongation of the pterotic. Externally the sensory canal is only protected by 

 ligament. The tenth dermal tubule enters the canal at about the middle of the 

 sphenotic, and in front of the canal in that bone transmitting the otic branch of the 

 outer buccal nerve. 



Squamosal or Pterotic. — The iufra-orbital canal, leaving the sj)henotic and still passing 

 backwards and somewliat downwards, enters on the pterotic, which supports it for the 

 last 30 mm. of its length, until, between the pterotic and the succeeding supra-temporal, 

 the infra-orbital canal anastomoses with the lateral or body canal. A more or less perfect 



18* 



