OCR Text |
Show 44 MR. A. D. IMMS ON THE DENTICLES [Jan. 17, pavement of denticles very closely packed together. A similar investment covered the branchial arches up to the bases of the gill-filaments. The specimen examined measured 56 cm. in length. 4. M ustelus ljevis Rond.-In an example measuring 150 cm. long the distribution of denticles over the mouth and pharynx was similar to that in Sphyrna. It will be seen on referring to PI. III. fig. 1, that the denticles of this species are rhomboidal in shape with rounded angles, and each has a well-defined basal plate. They are very closely packed together, so that each partially overlaps two or more of its fellows immediately behind. Their basal plates are also seen to closely interdigitate with one another. 5. Galeus (Galeorhinus) canis Rond.-Denticles in this species have a distribution identical with that found in Sphyrna and Mustelus. In PI. III. fig. 2 is represented a strip of the mucous membrane of the pharynx with the denticles in situ. It will be observed that they are closely and regularly arranged together, but no imbrication takes place as in Mustelus. The fish examined measured 155 cm. in length. Fam. L amnid.e . 6. L amna (Ox y r h in a ) cornubica Gmelin.-In a specimen measuring 79 cm. in length the denticles had a distribution identical with that found in the three preceding genera. Fam. N otidaxid.e . 7. N otidanus (H eptanchus) cinereus Gmelin.-In an example of this species measuring 65 cm. in length denticles were found to be generally distributed over the lining of the mouth and pharynx, and also to extend over the pharyngeal edges of the branchial arches. They were absent, however, from the anterior and posterior faces of the latter. In form they are characteristically tricuspid, as is represented in PI. III. fig. 3, and are identical in all respects with the placoid scales of the skin. Fam. S cylliid.e . 8. Scyllium canicula L.- After an examination of a number of examples of this fish, which were used for class demonstrations, no denticles were to be detected over any part of the lining of the mouth or pharynx. 9. C hiloscyllium indicum Gmelin.-In a specimen 34 cm. long denticles were found scattered somewhat irregularly over the lining of the mouth and pharynx, but they did not extend on to the branchial arches. 10. P ristiurus melanostomus (Rafinesque) Blainv.-In a very young individual, 14 cm. in length, oral and pharyngeal denticles |