1895.] SENSOEY CASTAl SYSTEM OF FISHES. 291 



region, and some little distance in front of this a lateral branch 

 passes off — the operculo-mandibular branch. The canal then 

 makes a slight ventral curve, and posterior to the orbit di\ddes into 

 supra- and sub-orbital branches. 



The Supra-orhital Branch, like the main canal, is conducted 

 in its course by a series of small drainpipe-like canal-bones. It 

 passes forwards and inwards to a point directly above the orbit, 

 and then makes a curve towards the nasal capside, dorsal and 

 anterior to which it terminates by a minute pore. 



The Suh-orhital Branch passes from the main canal on the lateral 

 border of the frontal in a series of canal-bones forming the circum- 

 orbital series. These conduct the branch around the inferior and 

 posterior borders of the orbit, the branch terminating ventral and 

 slightly posterior to the nasal capsule. 



The Operculo-mandibular Branch. — Leaving the main canal in 

 the inferior and posterior portion of the pterotic, the operculo-man- 

 dibular branch passes into a small tube of bone which traverses 

 the region between the pterotic and preoperculum, along the 

 superior and posterior borders of the hyomandibular bone. Parker 

 and Bettany regard this small tube-like bone as a supra-temporal. 

 It cannot, I think, be regarded as otlier than one of the drain- 

 pipe-like canal-bones which conduct the main canal and branches 

 over the greater part of the skull, and homologous with those 

 figured and described in Amiurus eatus (see p. 280). Entering the 

 apex of the preoperculum, the canal traverses the anterior border, 

 passing through the whole length of the bone. In its course 

 it gives off fine branches, which are distributed over the inferior 

 border of the bone ; a number of finer branches pass off over 

 the superior portion of the preoperculum. Leaving the preoper- 

 culum, a small dermal connection conducts the branch into the 

 articular portion of the mandible, entering it on its lateral border. 

 It passes downwards and forwards and into the dentary, traversing 

 the ventral portion of the same. In the anterior portion of the 

 dentary the branch divides in the substance of the bone, anasto- 

 mosing previous to opening at a terminal pore. There is no 

 connection between the two branches of either side. 



3. The Occipital Commissueb leaves the main canal on the 

 inner border of the pterotic, and makes an anterior flexure over the 

 region of the supra- occipital, and meets with the main canal of the 

 opposite side. 



I have not figured or described the innervation, as the distri- 

 bution of the cranial nerves is almost identical with that of &ox 

 lucius. 



Variation. 



In the Salmon there seems to have once been a series of canals 

 passing through ' certain bones of the head which have been 

 replaced by the system borne by the series of drainpipe-like canal- 

 bones already described. 



In young examples a series of canals are easily traced in the 

 parietal, frontal, supraethmoid (Parker), and nasal bones. These 

 canals vary a little in different specimens, but, generally speaking, 



19* 



