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Show 1905.] OF ELASMOBRAXCU FISHES. 43 djeddeusis, Raja clavata, Torpedo marmorala, and Tryyon sephen, together with Chimcera monstrosa. The paper is accompanied by numerous figures which show the varied forms assumed by these denticles. Those of Heptanchus in nowise differ in their form from the placoid scales of the skin in that genus. Those of Mustelus and Pristis are regarded as being intermediate in form between the teeth of the respective species on the one hand and the placoid scales 011 the other. In other genera their resemblance to the placoid scales becomes more remote. The author briefly refers to the function of these structures, and suggests that they may serve in grinding up the food. For that purpose a slight side rubbing-movement of the jaws and gill-apparatus might suffice. By means of such a motion the scaly mucous membrane would act like two rough surfaces. Mention is also made of the difficulty to account for the presence of such structures over an area like the pharynx, which is of hypoblastic origin. He shares the view of Hertwig that their presence in that region is more likely due to a migration of the ectoderm rather than to the possibility of the hypoblast having acquired a scale-forming capacity. My observations on the presence and distribution of oral and pharyngeal denticles have been made on specimens belonging to eighteen genera of Elasmobranchs. A. S e l a c h o i d e i *. Fam. Oarcitarud.e. 1. Carcharias glaucus Rond.-In an individual which measured 39 cm. in length, denticles were present over the floor of the oral cavity and along the pharyngeal margins of the branchial arches. Steinhard remarks that in an example of this species which he studied, measuring 46 cm. long, the whole of the cavity of the mouth and pharynx, together with the branchial arches, were covered with denticles which extended as far back as the commencement of the oesophagus. 2. C. laticaudus Midi. & Henle.-An examination of four specimens of this species, varying in length from 18-42 cm., showed that the oral cavity was closely covered with minute denticles, which extended backwards to about the level of the first gill-clift. In the region of the pharynx they were found only along the inner or concave margins of the branchial arches. In a fifth example of this fish, 26 cm. long, no denticles were to be detected except a few along the branchial arches. 3. S phyrna (Z ygjsna) malleus Risso.-In this species the whole of the lining of the oral and pharyngeal cavities, as far back as the entrance into the oesophagus, was covered with a complete * In this classification I have followed Giinther, Brit. Mus. Cat. l‘ ishes; vol. viiu 1870. |