SCALES 



I8 7 



disadvantages, and would certainly impose some restriction on the 

 lateral flexures of the body in swimming, and hence in the different 

 groups of Fishes it may happen that, in the more specialised forms, 

 an imbricated cycloid squamation supersedes a rhombic condition, 

 and with the change the Fish acquires greater lateral mobility. 

 Even in the same Fish the gradual substitution of the cycloid for 

 the rhombic type may be observed. In ihe*A\isiralian Aetheolepis, 1 

 a fossil genus related to the European Liassic Dapedius, there is a 

 gradual transition along the sides of the body between the articulated 

 rhombic scales of the relatively immobile trunk and the cycloid 

 overlapping scales of the flexible tail ; and it may be mentioned 

 that, even where a typical rhombic squamation exists, the peg-and- 



FIG. 102. Acipenser ruth- 

 enus. A, Side view of 

 the trunk of a speci- 

 men 30 cm. in length 

 (nat. size) ; d, dorsal 

 row of plates ; I, I', 

 lateral rows ; between 

 the rows of large 

 scutes may be seen the 

 numerous small den- 

 ticles which are repre- 

 sented ( x 10) in B ; C, 

 one of the large scutes 

 ( x 10). (From Hert- 



socket articulation may be wanting in the caudal region, so as to 

 ensure greater freedom of movement. Mechanical considerations 

 may also explain the overlapping of cycloid scales. From the 

 mode of attachment of the myocommata to the dermis, the con- 

 tractions of the myotomes, through the pull which they exert on 

 the former, tend to deflect or depress the scale-areas, particularly 

 at their anterior margins. 



In the surviving Crossopterygii, as in Polypterus, the scales 

 are rhomboidal and thick, and they only slightly overlap. They 

 articulate with one another by nieans of marginal peg-and-socket 

 articulations (Fig. 106, B). A thick layer of hard, glistening, 

 enamel-like substance or " ganoin " forms the outer layer of the 

 scale ; the inner layer consisting of bone in which dentinal 

 tubules as well as bone-cells are present. In the numerous fossil 

 members of the group the scales are either rhomboidal or cycloid. 



1 Smith Woodward, op. cit. p. 449. 



