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Show 1905.] OF ELASMOBRANCH FISHES. 47 largeur cles arcs, la place des dents pharyngiales inferieures et partiellement la place des dents pharyngiales superieures sont couvertes de tr&s petites dcailles rudes et pointues, la pointe dirigee en arriere." On Echinorhinus he remarks, " Appendices* deux cotes l er, 2e, 3e, 4e et cote exterieur 5e arc, longs sans dents, long 6 mm." .......... " Pas de dents pharyngiales inferieures a voir. Pas de dents pharyngiales superieures a voir." In his account of the primitive Shark Chlcimydoselachus an-guineus, Garman f mentions that both the mouth and throat of that fish are covered with scales which are largest on the inner edges of the gills. It will be noted from the foregoing account that the presence of denticles in the lining of the mouth and pharynx is of very wide distribution among the Elasmobranchii. Out of the nineteen species which I have examined, only five were found to be totally devoid of them. If there be added to these the species examined by Steinhard and Popta we have a total, with Chlamydo-selachus, of thirty-two species, out of which only nine (or about 28 %) have no denticles whatever. The facts and conclusions that are to be gleaned from a study of these denticles may be summarised as follows :- 1. In Heptanchus cinereus, Chlcimydoselachus anguineus, Mus-telus Icevis, Galeus canis, Sphyrna malleus, Lamna cornubica, and (probably) Rhinobatusproductus, denticles are uniformly distributed over the whole of the mucous membrane of the mouth, pharynx, and branchial arches, and extend backwards to the commencement of the oesophagus. Since this condition is met with in such generalised types as the two first named genera, there is good reason to believe that it represents the primitive method of distribution which has been inherited by them from the ancestral forms of existing Elasmobranchs. It seems probable that the variations in the distribution of the denticles which are met with in other species have been derived from this condition through their becoming restricted to certain areas only. The first and simplest modification is exhibited in Acanthias vulgaris. In this species the denticles are wanting from the roof of the mouth and pharynx. In Alopecias vulpes these structures are absent from both the roof and floor of the mouth and pharynx, and hence they are restricted to the pharyngeal margins of the branchial arches. In Rhina squatina they have ceased to be developed in the pharynx except on the mucous membrane covering the hyoid and first branchial arches. They are retained, however, over a considerable area on both the roof and floor of the oral cavity. In Scyllium canicula, Echinorhinus spinosus, Myliobatis aquila, Torpedo ocel-lata, and Trygon walga denticles have become lost altogether. 2. In all cases where I have examined the denticles microscopically, their structure and form proved that they were uii- * I. e. gill-rakers. + Bull. Mus. Comp. Zool. Harvard Coll. vol. xii. No. 1, 1885, p. 2, pi. v. |