. The Cambridge natural history. Zoology. i86 FISHES 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 { 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
. The Cambridge natural history. Zoology. i86 FISHES 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 { 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 ^ Scales bear a segmental relation to the subjacent muscle-segments or myotomes, sometimes being disposed in oblique transverse rows. Fig. 101.—Development of a scale in L&]n- dostexis osseus x 330. Basal plate, with included bone cells, at first distinct from the spine; e, enamel; , enamel organ ; epj epidermis, with large gland cells ; p, dermic papilla which forms the ves- tigial spine ; Sd, scleroblasts. (From Klaatsoh.) 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 '
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
