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Show 1905.] OF ELASMOBRANCH FISHES. 45 were entirely absent. In two much larger specimens (47 cm. and 58 cm. long respectively), studied by Steinhard, denticles were found to be present on the mucous membrane investing the gill-arches. Fam. H eterodontid^e . 11. H eterodontus (C estracion) philippi (Bl.).-In a museum specimen, measuring 60 cm. in length, coarse denticles were present on the roof and floor of the mouth and pharynx, but as the specimen was not available for dissecting purposes I was unable to determine the precise limits of their distribution. Fam. SriNACiiLE. 12. Centrina salviani Risso.-In an example 23 cm. long oral and pharyngeal denticles were found to be entirely wanting. 13. A canthias vulgaris Iiisso.-An examination of several specimens of this fish, whose length averaged about 60 cm., showed that denticles were present over the floor of the mouth and pharynx, and extended from the latter on to the mucous membrane covering the branchial arches, where they extended as far as the bases of the gill-filaments and even over the gill-rakers also. In a young fish, 26 cm. long, the denticles had not yet appeared above the surface of the mucous membrane, with the exception of a small patch over the region of the basi-branchial cartilage. A portion of the mucous membrane, from the floor of the pharynx of this species, containing denticles, is represented in PI. III. fig. 4. Fam. R hinidje. 14. R hina squatina L -In an example of this species measuring 80 cm. long denticles were found sparsely scattered in an irregular manner over the roof and floor of the oral cavity, and they extended also on to the pharyngeal margins of the hyoidean and first branchial arches. The denticles of this species are very remarkable in their form (vide PI. III. fig. 5). Each consists of a large basal plate, irregular in its outline, and in its centre is a boss-like protuberance which has its surface intersected by several blade-like ridges. The protuberance appears to be the last remnant of the spinous portion of the denticle which attains its full development in the placoid scales of the skin. Steinhard deals with this species in considerable detail, and he regards the denticles as being placoid scales which have not reached their full development owing to an insufficient supply of lime salts. B. B a t o i d e i . Fam. R hinobatidje. 15. R pinobatus productus Girard.-In this species, closely arranged denticles completely invest the lining of the mouth and |