[18] Vergl. Anat. der Wirbelth. Leipzig, 1898, p. 497.
[19] To this category are perhaps to be referred cartilaginous pieces occurring in the Rabbit, Mus and Sorex (see Fig. 29 above).
[20] "On the Coracoid of the Terrestrial Vertebrates," P.Z.S. 1893, p. 585.
[21] Horny matter is apt to be formed upon extremities; instances which are well known are the "claws" upon the tail of the Lion and Leopard and the Kangaroo Onychogale. For an account of the first see Proc. Zool. Soc. 1832, p. 146.
[22] Cf. Tomes, A Manual of Dental Anatomy, 5th ed. London, 1898.
[23] Materials for the Study of Variation, London, 1894.
[24] Morph. Jahrb. xix. 1892, p. 502.
[25] It would be of the greatest interest in relation to this and many other problems to ascertain the precise meaning of the monophyodont dentition of Ornithorhynchus.
[26] Proc. Zool. Soc. 1899, p. 922.
[27] Mr. M. Woodward, however (P.Z.S. 1893, p. 467), is disposed to think that in some Macropodidae at any rate the supposed tooth of the second set really belongs to the milk dentition, arising late between Pm_{3} and Pm_{4}.