186 FISHES CHAP. 
the primitive rhomboidal shape of the dermal denticle is retained ; 
in others-a rounded or cycloid scale supplants the earlier rhombic 
type. Within the limits of the same group (e.g. Crossopterygii) 
there are examples of the independent evolution of a cycloid from 
a pre-existing rhombic squamation ; and with the introduction of 
the cycloid type an overlapping or imbricated disposition of the 
scales always takes the place of the marginal articulation of the 
rhombic type. 
As to the causes which may have determined the shape and 
mutual relations of scales interesting suggestions have been made.! 
Scales bear a segmental relation to the subjacent muscle-segments 
or myotomes, sometimes being disposed in oblique transverse rows 
of a scale in Lepi- 
eXs---- op, dosteus osseus X 330. 
NE b.p, Basal plate, with 
: 7) @ included bone cells, 
ONarS ae S/N Lp. at first distinct from 
ie Xi “Tota 
Ne ae the spine ; e, enamel ; 
ie e.0, enamel organ ; 
: © 5S) » 2/0 <6 8 
Prey eee aoe us ean tele e( Fic.101.—Development 
BYoryyNe e 
woe Sar a Oo 
\ 
\o) 
On 
aoe “acd ep, epidermis, with 
Woe . 
ee LeCKS large gland cells ; 
p, dermic papilla 
which forms the ves- 
tigial spine; Sci, 
scleroblasts. (From 
Klaatsch. ) 
Sass 
Se Gee eee 
—— =o 
coinciding with the latter, or the rows may be so far increased as 
to be multiples of the myotomes. From mechanical considerations 
depending on the sigmoid shape and interdigitating relations of 
the myotomes and their separating fibrous septa or myocommata, 
and the attachment of the myocommata to the dermis, the con- 
traction of the myotomes during the lateral flexions of the trunk 
in swimming has a tendency to wrinkle the skin into definitely 
circumscribed rhombic areas, thus determining the shape, limits, 
and disposition of the scales which are developed in those areas. 
The rhombic was probably the primitive shape of scales, and is 
certainly characteristic of the palaeontologically older types of 
scaly Fishes. Generally the rhombic condition is associated with 
a peg-and-socket articulation between the upper and lower margins 
of adjacent scales. But a rhombic squamation is not without 
' Ryder, Proc. Acad. Nat. Sci. Philadelphia, 1892, p. 219; Smith Woodward, 
Nat. Sct. iii. 1898, p. 448. 
