. The Cambridge natural history. Zoology. SCALES 187 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 Aetheolejpis} a fossil genus related to the European Liassic Dapedius, there is a gradual transit
. The Cambridge natural history. Zoology. SCALES 187 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 Aetheolejpis} 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- PiQ. 102.—A cipenser ruth- enus. A, Side view of the trunk of a speci- men 30 cm. iu 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- 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 weW as bone-ce
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
