192 FISHES CHAP, vii 



Fish. 1 The formation of these rings depends on the fact that 

 the lines of growth on the surface of the scale are more widely 

 separated from one another on that portion of the scale formed 

 during summer, and relatively closer together on that part which 

 is formed during the winter ; the more rapid growth in the warmer 

 season probably being due to favourable conditions as to food 

 and temperature, and the retarded growth of the colder season to 

 the reverse. Hence, by counting the alternating zones of close- 

 set winter lines and less closely approximated summer lines of 

 growth, a reliable clue may be gained as to the age of the Fish. 



In the Dipnoi, 2 as in Teleosts, the scales are enclosed in 

 dermal pockets, and exhibit a regular, imbricated disposition in 

 oblique rows (Fig. 304, A). In shape they 

 are nearly cycloid, or slightly oval, with 

 the long axis coinciding with that of the 

 body. Structurally, also, they bear some 

 resemblance to Teleostean scales, although 

 differing in details. On the outer surface 



Fid. 110. Hypostoma com- , , 



mersonii. A scale from of the scales there are numerous small 

 the periphery of the con i ca i sp i ne s. No significance, other than 



caudal fin, x 50 ; one of 



the spines (s) is im- as an example of evolutionary convergence, 

 planted in its socket (s'). b attached to the resemblance between 



(From Hertwig.) 



the scales of Fishes so widely separated as 

 the Dipnoi and the Teleosts. 



All known fossil Dipnoi had scales of a similar character, 

 although differing greatly in size in different genera. In some 

 (e.g. Dipterus) a layer of enamel -like substance invests the 

 exposed portions of the scales. 



1 Hoffbauer, "Die Altersbestimmung des Karpfen an seiner Schuppe." Jahresb. 

 des Schlesischen F-ischerei- Vereins, 1899 ; J. Stewart Thomson, Journ. Marine Biol. 

 Assoc. vi. No. 3, 1902, p. 373. 



2 Giinther, Phil. Trans, clxi. 1871, p. 516 ; Klaatsch, op. cit. p. 209. 



