1907.] THE SCALES OF FISH. 765 



until stages intermediate between the cosmine and the denticle 

 have been discovered. 



We have determined what are the three chief kinds of scale ; 

 let us now see of what importance they are in classification. 



The Systematic Importance of the Scales in the 

 Dipnoi and Teleostomi. 



The CosMOiD ty23e and its derivatives. — That type of scale 

 which we have called cosmoid occurs only in the extinct Osteo- 

 lepidoti (Crossopterygii) and in the Dipteridse. In the thick 

 rhombic scales of the Osteolepidfe it is most perfectly shown. The 

 scale of Megalichthys has been fully dealt with aloove (p. 755). 

 That of Osteolepis is quite similar, as may be seen in Pander's 

 excellent figures (15). A section of the edge of the scale 

 of Osteolepis is here figured, showing the outer layer of typical 

 cosmine and the absence of stratified ganoine (PI. XLIII. fig. 5). 

 Diplopterus difiers scarcely from Megalichthys (Williamson). 



Now it is extremely interesting to find that the scale of Diptertis 

 has exactly the same structure, for it has long been recognised 

 that the Dipteridae approach the Osteolepidoti (Crossopterygii) 

 more closely than any other group of fish. The importance of this 

 fact is enhanced by the knowledge that the cosmoid type occurs 

 in no other Order. Pander's figures leave no doubt about the 

 resemblance of the scale of Dipterus to that of Osteolepis. 

 Unfortunately I have not had material favourable for sections ; 

 but the fragments I have examined confirm his descriptions. 

 This cosmine layer is of such peculiar, elaborate and complex 

 structure, that we cannot suppose it to have been independently 

 developed in the two cases. There seems no escape from the 

 conclusion that the common ancestor of the Osteolepidte and 

 Dipnoi had scales of this cosmoid type. 



The evidence with regard to the other families of the Osteo- 

 lepidoti (Crossopterygii) is not so clear-. Various forms of scale 

 occur among them ; some of which are probably degenerate 

 cosmoid scales, while others perhaps belong to some difiereni 

 though related type. 



In the Holoptychiidse the scales are usually rounded, and 

 ornamented with tubercles and ridges. Williamson (see his 

 fig. 24, plate 42) figures a section through the scale of Holoptychius 

 showing an outer layer which is undoubtedly a stage either in the 

 formation or in the degeneration of the typical cosmine structure. 

 According to Rohon's careful account, the scale of Iloloptychiits 

 (probably another species) is very like that of the Osteolepida?, 

 excepting for the outer layer. Instead of being built in the 

 uniform and beautifully regular manner of typical cosmine, it is 

 in the form of the irregular tubercles and ridges already 

 mentioned ; and these are composed of dentine-like tissue, 

 resembling the cosmine layer of the palseoniscoid scale. There is 

 no alternating svstem of pulp-cavities and chambers, but merely 



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