192 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 
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 
ae Lid Saapeiea oe differing in details. On the outer, surface 
mersonii, A scale from Of the scales there are numerous small 
er ea apenas conical spines. No significance, other than 
the spines (s) is im- as an example of evolutionary convergence, 
pene eee (s)} can 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. Dipterus) a layer of enamel-like substance invests the 
exposed portions of the scales. 
Sa 
1 Hoffbauer, ‘‘ Die Altersbestimmung des Karpfen an seiner Schuppe.” Jahresb. 
des Schlesischen Fischerei-Vereins, 1899; J. Stewart Thomson, Journ. Marine Biol. 
Assoc. vi. No. 3, 1902, p. 373. 
? Giinther, Phil. Trans. clxi. 1871, p. 516; Klaatsch, op. cit. p. 209. 
