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Show 5 6 DR. W. G. RIDEWOOD ON THE CRANIAL [May 3, Ectosteal and Endosteal Bones.-With regard to the relation between ectosteal and endosteal ossifications in the skull of bony fishes, there is reason for believing that the ectosteal is the more primitive, and that even in the few cases in which related ectosteal and endosteal bones remain distinct, as in the postfrontal and sphenotic of Amia*, the endosteal and ectosteal parts of the articular bone in Amia, Lepidosteus +, Arapaima, Albula, Elops, Megalops, Hyodon, and Gymnarchus, the endosteal and ectosteal parts of the mesethmoid of Megalops, and those of the glossohyal in a great variety of forms, the endosteal ossification has been set up in sympathy with the ossification taking place in the dermal tissues. The process of ossification is infectious, if one may employ such a term in this connection, and the increase of blood-supply, and the redistribution and alteration in the character of the cells and matrix in the one part is shared by the subjacent parts to a greater or less degree. As other examples of such related ossifications, there may be mentioned the squamosal and pterotic, the postfrontal and sphenotic in Teleosteans, the prefrontal and par-ethmoid and the ectosteal and endosteal parts of the angular in such forms as Lepidosteus and Arapaima, in which the upper part of the bone articulating with the quadrate, is endosteal, while the ventral surface of the bone is sculptured, and has all the appearances of a dermal bone. Having regard, therefore, to the superior antiquity of the dermal constituents of the combined bones, whether these have arisen phylogenetically around sensory canals or by coalescence of integumentary denticles-a matter ably discussed by Allis (Journ. Morph, xiv. 1898, pp. 426-431)-it is preferable to adopt the names that belong to such superficial ossifications, e.g. squamosal J, prefrontal, postfrontal §. And * An occasional feature only, although Traquair (‘ Ganoid Fishes Brit. Carb. Form.,' Palseont. Soc. 1877, p. IB) and Allis (Journ. Morph, ii. 3, 1889, p. 479) seem to regard it as constant. f Van Wijhe (Nied. Arch. f. Zool. v. 3, 1882, pp. 268 & 281) regards thecoronoid also of Lepidosteus and Amia as consisting of separable endosteal and ectosteal elements, which he calls the autocoronale and the dennocoronale or suprangulare. The autocoronale of Lepidosteus, however, is either a bone which is to be identified with the sesamoid articular (see p. 72), or is merely a thickened part of the splenial. In neither Lepidbsteus osseus nor L.viridis have I been able to separate it from the splenial, yet Van Wijhe (I. c. pi. 16. fig. 9, a.c.) figures it as a separate bone. The autocoronale of Amia, on the other hand, the bone which is marked d by Bridge (Journ. Anat. & Phys. xi. 4, 1877), is a special nodule of bone developed in Meckel's cartilage in relation with the articulation between the symplectic and the mandible. (The bone a of Bridge is the angular; b and e, which I have never seen as two separate bones, are the endosteal articular; while the ‘ angular" of Bridge is the ectosteal articular.) J In his description of the skull of QrammicoUpis, Shufeldt (Journ. Morph, ii. 2, 1889, p. 280 & fig. 2) discriminates between the squamosal and the pterotic. He says: " At the distal extremity of the squamosal I detect a small, flake-like piece of bone, thoroughly attached, though individualised by sutural traces, which I take to be the representatives ot the pterotic. Since, however, he also letters the'squamosal and pterotic separately in his figure of such a well-known skull as that of Caranx (fig. 6, p. 285), it would appear that no great importance need be attached to the distinction. § Cole & Johnstone (Proc. & Trans. Liverp. Biol. Soc. xvi. 1902, pp. 160-161), while admitting the propriety of using the term squamosal for the dermal bone lying |
