. Chordate morphology. Morphology (Animals); Chordata. Sharpey's fibei B vascular canal Figure 8-34. Sections through the scales of paloeoniscoids. A, Orvikwna vardiaensis (Upper Middle Devonian) in which the ocellulor base has many branching canals of Williamson; B, £/onicht/iys punctafus with a cellular base and a laminated ganoin cover. (A, after 0rvig, 1957; B, after Aldinger, 1937) ganoin layers dentinal tubules. Figure 8-35 Williamson's canals. canals of Williamson Scale of a perleidid, the iepisosteid type having type, occurring already in the Lower Devonian genus Chei- roiepis, and it
. Chordate morphology. Morphology (Animals); Chordata. Sharpey's fibei B vascular canal Figure 8-34. Sections through the scales of paloeoniscoids. A, Orvikwna vardiaensis (Upper Middle Devonian) in which the ocellulor base has many branching canals of Williamson; B, £/onicht/iys punctafus with a cellular base and a laminated ganoin cover. (A, after 0rvig, 1957; B, after Aldinger, 1937) ganoin layers dentinal tubules. Figure 8-35 Williamson's canals. canals of Williamson Scale of a perleidid, the iepisosteid type having type, occurring already in the Lower Devonian genus Chei- roiepis, and it is generally assumed that those with the canals (the lepidosteid type) were derived from those with- out. However, Orvikutna (Figure 8-34) of the Middle De- vonian already had a scale suggesting the lepidosteid tyjse (Figure 8-37). The base of the scale has numerous ascending canals which show many side branches. These canals do not reach the pulp cavities of the dentine and appear to house only the osteoblasts of the acellular bony base. The early appearance of such a modified scale suggests an early radi- ation of scale types among the palaeoniscoid fishes. It is unlikely that the scale of Orvikuina is related to that of Lepisosleiis. The lepidosteid type may have been achieved independently by many kinds of fishes at different times ( a case of parallel or convergent evolution). It is also possible that both types of scales (with and without Wil- liamson's canals) arose at the same time and have continued down to the present, each radiating into a number of sub- types. SCALES 235. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Jollie, Malcolm. New York, Reinhold
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