. The elasmobranch fishes . A. BJiinohatis producius. (Chester Stock, orig.) iW^ ho. B. Keierodontus francisci. (Duncan Dunning, del.) Fig. 77. Transitional vertebrae. hd., dorsal basal (basidorsal) plate; hv., ventral basal plate; , foramen for dorsal root nerve; , foramen for ventral i-oot nerve; iv., ventral intercalary; (and) sid., neural spine; r., ril). regions is not so clear; while in still more specialized forms tlie column is more or less clearly separated from the cranium, attachment of the two being made by processes of the cranium and the column (rays). This segment o
. The elasmobranch fishes . A. BJiinohatis producius. (Chester Stock, orig.) iW^ ho. B. Keierodontus francisci. (Duncan Dunning, del.) Fig. 77. Transitional vertebrae. hd., dorsal basal (basidorsal) plate; hv., ventral basal plate; , foramen for dorsal root nerve; , foramen for ventral i-oot nerve; iv., ventral intercalary; (and) sid., neural spine; r., ril). regions is not so clear; while in still more specialized forms tlie column is more or less clearly separated from the cranium, attachment of the two being made by processes of the cranium and the column (rays). This segment of the column (fig. 76a, Heterodontus), unlike that of Hep- ianchus, is nsnally clearly divided into centra (c.). In practically all forms the relation of the dorsal and ventral arches to the centra is equally clear. In a type like Pseudotriacis, how^ever, the segmentation is irregular, both in the centra and in the neural arches of the column. This irregularity is especially marked in the most anterior vertebra which has fused into a solid ring. In the rays (Rhinohatis, fig. 76b) the larger part of this anterior region has second- arily fused into a solid vertebral plate, the segmentation of which is made out only through a study of the foramina and certain lateral processes.
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