1 92 FISHES CHAP. VII 



Fish.-' 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," as in Teleosts, the scales are enclosed in 

 dermal pockets, and exhibit a regular, imbricated disposition in 

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



^^^Ji^g^^^g* are nearly cycloid, or slightly oval, with 

 ^^E:i%_Jl^^^glSl the long axis coinciding with that of the 

 ^^S^"^^i?^^°^^S^« body. Structurally, also, they bear some 

 ^ <fe^^^ | .^'^ '^ ^ ^^ resemblance to Teleostean scales, although 



s' . ■ ' & 



,,„ ' differing in details. On the outer surface 



Fig. 110. — Eyposioma com- 0,11 



mersonii. A scale from 01 the scales there are numerous small 

 the periphery of the gonical spines. No significance, other than 



caudal nn, x 50 ; oue of ^ . 



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

 fFrom Heriwig°f '* '''^' ^an be attached to the resemblance between 

 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. Dijpterus) a layer of enamel -like substance invests the 

 exposed portions of the scales. 



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

 des Schlcsischen Fiseherei- Vcreins, 1899 ; J. Stewart Thomson, Journ. Marine Biol. 

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



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



