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Show 1896.] MAMMALIAN DENTITION. 559 represented in one set, belong to the replacing series, being preceded by tooth-vestiges referable to the milk-dentition. This last conclusion I can now entirely confirm and strengthen, for the specimens which I have investigated exhibit these features much better than Leche's embryos did1. Both Leche and myself have investigated Erinaceus, Ericulus, Sorex, and Talpa, and he has further studied Crossopus, Scalops, and Condylura, whilst I have independently investigated Centetes and Gymnura. These are unfortunately representatives of only 5 out of the 9 families of living Insectivora (Flower and Lydekker, 4), thus leaving at least 4 other families, some of which are extremely interesting, still to be investigated. In addition to studying the relations of the two sets of teeth, I have attempted to trace the origin of the cusps of the molar teeth, noting especially the order of development of those structures in the light of the researches of Osborn, Rose, and Taeker. ERINACEUS EUBOPJEUS. Of our common English Hedgehog I have examined two specimens intermediate in age between Leche's stages E and E, that is between his oldest foetus and bis new-born young. Further, I have examined a large series of dried skulls, including those in the collection at the British Museum. The statements concerning the milk-teeth of this genus and allied forms in many, especially the older, text-books are most misleading, and even in more modern works we find the whole group described as diphyodont. This, though strictly true, was not based on any detailed examination of the various genera, but rather hastily concluded from the knowledge that one form was found to exhibit this condition, or else copied from some old and unreliable accounts, as, for instance, that of Rosseau (21), who stated that Erinaceus had a deciduous dentition composed as follows, viz.;-• i. |, pm. A, which were shed at the age of 7 weeks. Dobson (3) also speaks of a full milk-dentition, but it is obvious that he simply described as milk-teeth all those teeth which were visible in the jaw of tbe young Hedgehog at birth, and that he had never seen any actual replacement. He states, in contra-distinction to Rosseau, that at 6 weeks all the permanent teeth were present. The erroneous nature of these conclusions has been pointed out by Leche (9); and it may be ascertained by anyone, from the study of a few .young skulls, that the only milk-teeth recognizable by the ordinary methods of dissection or examination of dried skulb are i. f, c. (1), pm. \. The remainder being only to be made out, and then with difficulty, by the examination of serial sections of foetal jaws, a method not adopted by the earlier observers. 1 It is just possible that our English Hedgehog differs iu respect to the amount of development of these uiiik-tooth vestiges from its continental cousin, although they are considered as one species ; on the other hand, m y two specimens may represent individual variations. |
