058 MR. R. H. BURNE ON THE ANATOMY OF THE [May 25, 



which appear to be characteristic of families or even larger 

 groups. 



The nostrils are perhaps the most variable part, and also that 

 in which variation is correlated least with natural affinities. 



The position of the anterior nostril directly above the rosette 

 is almost universal, no doubt in order that the incurrent water 

 may play directly upon the olfactory membrane. This position 

 is also probably due in part to the close connection that there is 

 in almost every instance between the axis of the rosette and the 

 front lip of the nostril, which indicates that the rosette belongs 

 essentially to the anterior part of the olfactory chamber, "When 

 the rosette is elongated, as in the Eels, Siluroids, and some 

 Pleui-onectids (Rosette, Column II.) the nostril opens in front of 

 and not above it. 



The anteiior nostril is very frequently, especially in the lower 

 Teleostei, more or less tubular (Nostrils, Column II.). The tube 

 is particularly well marked in the Eels, some Siluroids, Anableps, 

 and Ophiocephalus, but the tendency towaixls tube-formation is 

 so widely disti^ibvited and variable in its occurrence and extent 

 that it probably has little to do with natural affinity. 



In certain groups, notably the Cyprinidfe and Gadidee, the 

 hinder wall of the tube is elevated to form a valvular flap 

 (Nostrils, Column III.), and in other gi-oups or separate genera 

 [Merhi^ccius, Esox, Salmonidfe, Clupeidse) this may be augmented 

 or replaced by a similar downward prolongation or curtain that 

 dips into the olfactory cavity above the centre of the rosette 

 (Nostrils, Column IV.). Both these developments of the bridge 

 of skin between the nostrils ai-e without doubt mainly of physio- 

 logical importance, although in restricted groups (e. g. Cyprinidai) 

 they are also constant enough in their occurrence to be of 

 systematic importance. 



Variations in the form of the posterior nostril seem also to 

 depend little upon affinity. 



Broadly speaking this nostril is either a simple open perforation 

 flush with the surface of the skin (Nostrils, Column VI.), which 

 may show considei^able differences in size, but commonly is either 

 circular, oval, or crescentic in shape ; or it is a slit or pin-hole 

 closed by valves. 



The crescentic type is highly characteristic of the Salmons, 

 Herrings, and Carps, though found also in Alerluccius, Esox, and 

 (in a bean-shape) in Zeus. It always more or less closely 

 embraces the hinder margin of the anterior nostril. 



The oval or circular form occurs in many groups, but can 

 hardly be regarded as characteristic of any. 



A valved condition (Nostrils, Column V.) is found chiefly 

 though not solely (some Siluroids) in fishes provided Avith acces- 

 sory nasal sacs, and forms part of a general mechanism for drawing 

 water forcibly into the olfactory chamber through the anterior 

 nostril. In their simplest condition the valves are merely the 

 thin converging lips of a minute perforation at the end of a short 



