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 lacunse 

 similar to those of bone are found *. Tomes i-egards 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 vasctdar channels 

 containing masses of pulp-structure as well as blood-vessels. In 

 vaso-dentine thei-e 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 t. 

 Apart from any knowledge of their mode of development, the 

 substance of the gill-rakeis 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 con- 

 nective 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 Cetorhinus the teeth are relatively greatly reduced in size, 

 and its food consists principally 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 cvirrent of water. As mentioned by Piof . Turner J, 



* Comp. Anat. vol. i. p 362. 



t " On the Structure and Development of Vaso-dentine," Phil. Trans. 1878^ p. 40. 

 Also Dental Anat., 2nd edition, pp. 88-92. 

 + Loc. cit. p. 275. 



