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Show 586 MR. M. F. WOODWARD O N [May 5 the base of this structure, the metacone and subsequently the hypocone being similarly derived from a backward extension of the base of the primitive dentinal germ. This primitive dentinal germ has, I believe, primarily a somewhat conical form in all cases and one of the cusps of the adult tooth appears to be the direct continuation of this primitive cone, the remaining cusps being outgrowths usually from its base. It is not customary to find a blunt expanded table-like dentinal germ from which the cusps arise as secondary outgrowths-a condition which, it appears to me, must be necessarily assumed to support Osborn's view that the protocone is primary but retarded and the paracone its lateral derivative accelerated. If it be the case that the paracone in the majority of Mammalia is the direct continuation of the primitive dentinal germ, and therefore of the single cone of the protodont mammalian ancestor, then we have the apparent anomaly of this primary cone giving rise, in the majority of forms, to the so-called paracone, i. e. the antero-external cone, while in a few it persists as the so-called protocone (antero-internal cone), a condition which suggests that the usually accepted identification of the cones of the upper molars is not in all cases the correct one. It may be possible that in the above too much stress is laid on the ontogeny of the molar cusps; but, on the other hand, do we know sufficient of the phylogeny, as deduced from palseontological evidence, to prove that the primitive cone has in all cases been correctly identified in the upper molars ? Por though we have, thanks to the researches of Owen (17), Osborn (16), and Marsh (11), knowledge of a great number of Mesozoic mammals, yet the molar teeth found are nearly all lower ones, and but few upper molars (save multituberculate ones) are known until we reach Tertiary times', when the teeth have assumed forms whose cusps can be more easdy homologized with those of living mammals than with the cusp or cusps of the Reptilian tooth or with that of the ancestral mammal. So that with regard to the evolution of the upper .molars we are almost completely in the dark, for we know of no Triassic or Jurassic protodont upper molars, but three maxillaa (I believe) containing triconodont teeth, and but a few which, according to Osborn, contain trituberculate teeth. I have tried to ascertain tbe exact number of upper jaws of Jurassic mammals possessing tritubercular molars or teeth approximating to that type, but have been unable to disperse the mystery wdiich seems to envelop them. In England wTe certainly possess one specimen, wdiich was described by Owen (17) as Peralestes longirostris, and is preserved in the British Museum ; with this Owen associated a lower jaw which is now separated by Lydekker (10) from this form and assigned to Amblotherium mustelula. Owen also described four upper jaws,which he referred to Stylodonpusillus; 1 Several isolated upper molars are known from the Upper Cretaceous rocks of N. America; some of these are said to be trituberculate (Osborn, " Mammals oftthe Upper Cretaceous Beds," Bull. Amer. Mus. JNat. Hist. 1893, p. 311), notably Bidelphops, but this, though triangular possesses at least 6 cusps. |
