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Show 1904.] OF THE SPOONBILL STURGEON. 31 The absence of such hard parts is to be correlated with their not being subjected to any use which would involve hard wear or much friction. In short, it is possible that the gill-rakers are to be looked upon as exoskeletal parts which were derived from an ancestral Selachian condition, where they exhibited little or no differentiation either in form or structure. Subsequently they became modified along lines of their own in order to fulfil particular functions, some migrating into the mouth to become teeth, while others passed on to the branchial arches and have given rise to the gill-rakers. From what I have described of their structure, the gill-rakers, at least their shaft portions, appear to be composed of a substance which bears a close resemblance to osteo-dentine, if not identical with it. Osteo-dentine was defined by Owen as that type of dentine in which the matrix is arranged around the vascular channels in the form of concentric rings, and in which lacunae similar to those of bone are found *. Tomes regards osteo-dentine as a substance which is developed by calcification proceeding through the interior of a pulp, and not by means of the calcification of a special layer of cells (odontoblasts) as is the case with other types of dentine. Consequently, in a tooth or structure composed of osteo-dentine there is no single pulp, but pulp and calcified tissue are quite inextricably mixed up, the vascular channels containing masses of pulp-structure as well as blood-vessels. In vaso-dentine there is a distinct pulp-cavity from which radiate canals which contain minute blood-vessels only. He further calls attention to the fact that in some teeth neither of the characteristics defined by Owen occurs, though, if their manner of development be taken into account, they are unquestionably made of osteo-dentine f . Apart from any knowledge of their mode of development, the substance of the gill-rakers of Polyodon bears a closer likeness to osteo-dentine than to any other structure, for the following reasons. It resembles that type of dentine in the absence of a common pulp-cavity, and in the nature of the anastomosing channels which contain one or more blood-vessels and some loose connective tissue (pulp-remains ?). The presence of bone-lacunte is an additional point of resemblance, though Tomes does not look upon it as being diagnostic of osteo-dentine, since they, or spaces very similar to them, are present occasionally in other kinds of dentine. In Getorhinus the teeth are relatively greatly reduced in size, and its food consists pi-incipally of minute surface organisms. The gill-rakers serve as a straining-apparatus which prevents the food-particles from passing into the branchial sacs with the outflowing current of water. As mentioned by Prof. Turner +, # Comp. Anat. vol. i. p 362. f " On the Structure and Development of Vaso-dentine," Phil. Trans. 1878, p. 40. Also Dental Anat., 2nd edition, pp. 88-92. J Loc. cit. p. 275. |