VIL SCALES I 87 
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 the Australian Aetheolepis,' 
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- 
Fia. 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; J, JU, 
lateral rows ; between 
the rows of large 
scutes may be seen the 
numerous small den- 
ticles which are repre- 
sented (x10) in B; C, 
one of the large scutes 
(x 10). (From Hert- 
wig.) 
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 means 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. 
