| OCR Text |
Show 82 MR. R. LYDEKKER ON A SIRENIAN JAW. [Feb. 2, the milk-molars were not like those of the present specimen, since the permanent molars were of a more Hippopotamus-like structure than are those of H. veronense, and show no indications whatever of selenodontism. Conclusive evidence of the Sirenian nature of our fossil is, however, afforded by the orbital region, which is almost exactly the same as in the Sirenian from Jamaica described by Sir R. O w e n 1 as Pro-rastomus sirenoides. In both we have the same peculiar eversion and projection of the lower border of the orbit which is absolutely characteristic of the Sirenia. In both, again, we notice the extraordinary size of the foramen for the exit of the fifth nerve, and its immediate proximity to the anterior border of the orbit, these being also distinctive Sirenian features. Then, again, a comparison of the last milk-molar with the upper molars of Halitherium schinzi (e.g. B.M. No. 40859) clearly shows the ordinal identity of the two forms. I take it, therefore, that the Sirenian nature of the Vicenza specimen is certain; and since its milk-molars agree in general structure and relative size with the permanent molars of Halitherium veronense, which occurs in the same country and probably on the same geological horizon, the evidence appears to be very strongly in favour of the reference of the specimen in question to that animal. Apart, however, from any question of specific reference, the specimen before us undoubtedly throws a flood of light on the origin of the Sirenia, and points clearly to their derivation from an ancestor belonging to an Artiodactyle Ungulate with short-crowned and selenodont molar teeth. It is, indeed, no new idea that the Sirenians show Ungulate affinities, this presumed origin having been very strongly urged by many zoologists; although Professor Flower, writing in the article " Manatee" in the ' Encylopsedia Britan-nica,' expresses his opinion that the few facts at present known relating to the ancestry of the Sirenians " lend no countenance to their association with the Cetacea, and on the other hand their supposed affinity with the Ungulata, so much favoured by modern zoologists, receives no very material support from them." If, however, m y interpretation of the affinities of the present specimen be accepted, it will go a very long way towards solving the problem of the Sirenian genealogy. So far as I a m aware, the component elements of the molar teeth of the Sirenians have not hitherto been homologized with those of mere typical teeth. The molars of the present specimen clearly show us, however, the homology of the elements of the simple and continuous transverse ridges found in Manatus and Halitherium schinzi, such ridges being clearly only one step more in the degeneration from a selenodont type exhibited in the molars of the specimen before us. I may add that although the upper permanent molars of H. veronense differ considerably from those of more typical species of Halitherium, while there is no evidence that the' latter had milk-molars of the type of the present specimen, yet I should not on 1 Quart. Journ. Geol. Soc. vol. xxxi. p. 559, pis. xxxviii., xxxix. (1875). |
