530 H, B. POLLARD, 



facialis (often wrongly called trigemini) or „Schädelhöhlenast". It 

 passes upwards intracranially to the parietal bone in which it lies 

 taking its course directly backwards near the depression in the centre 

 of the parietal. It supplies the mucous canal at the base of the 

 dorsal fin. 



Auchenaspis biscutatus, (Fig. 2.) 

 The head of a specimen 5 centimetres in length was cut in 

 sections 30 /n thick and a reconstruction of the nerves and organs 

 made as with Clarias. 



Auchenaspis is one of the African Siluroids and does not differ 

 greatly from Clarias though less primitive in many respects. The 

 canals are by no means so uniform in calibre as in Clarias and de- 

 generate in their terminal portions especially the supraorbital and 

 mandibular systems. Furthermore the pores are not always simple 

 apertures for in two cases a branch splits dichotomously. 



Infraorbital canal. Near the anterior edge of the snout 

 about half way between the middle line and the base of the anterior 

 barbule may be seen two pores. The most external is the first pore 

 of the infraorbital system. The second pore of this system is situated 

 at some little distance further back and more medianly the tubule 

 leading to the first pore curving outwards and forwards and that to 

 the second inwards and backwards. Between the two and slightly 

 nearer the second lies the first sense organ. A very small dermal 

 bone encloses the canal in this region just above the anterior end of 

 autopalatine. The second sense organ occurs only a short distance 

 posterior to the first. It is surrounded by a thin ring of bone formed 

 around the canal. The third pore opens in front of and external 

 to the nasal opening. From this point the canal curves gracefully 

 out to the fourth pore which occupies a corresponding position in re- 

 lation to the eye. At the middle of the curve lies the third sense 

 organ here also only enclosed in a thin ring of bone. So far the 

 canal has mainly been protected by membrane alone. Beyond the 

 fourth pore the direction of the curve is reversed and the canal passes 

 below the eye. The fifth pore lies opposite the centre of the eye and 

 the sixth is situated at the end of a short branch which arises di- 

 rectly posterior to the eyeball. Until it enters the frontal bone near 

 the origin of the latter branch the suborbital canal is never enclosed 

 in more than rudimentary bone. The fourth and fifth sense organs lie 



