| OCR Text |
Show 1896.] MAMMALIAN DENTITION. 563 region, thus resembling the condition which I described in the Macropodidae (28. p. 467). The antero-external cusp (protocoue of Scott) of dpm. 4 develops first, the antero-internal or deuterocone second, and the tetaro-cone third, the tritocone being wanting. The Molars. in 1 ni 2 Of the three molars of the adult, - ^ - - are alone developed in. 1, in. 2 ' in my specimens. The enamel-organs of these two teeth, both above and below, exhibit slight lingual continuations of the dental lamina; consequently these teeth do not develop in connection with the most deeply-seated portion of the dental lamina, but in relation to that situated nearer to the surface of the gum. The presence of this lingually-placed continuation of the dental lamina indicates that there is latent in the jaw the structure essential for the production of a second set of molars. In addition to this lingual growth, we find also a slight but constant labial outgrowth from that portion of the dental lamina connecting the enamel-organ of the functional molar with the oral epithelium. If this labial growth be compared with the vestiges of the milk-dentition seen in connection with i-_3 and 172, it is found that it is impossible to distinguish these structures from one another, they being precisely similar in their relations to the dental lamina and to the adjacent teeth, differing only in the fact that the labial growth connected with the molars is the most reduced. W e find, then, in the molar region indications of three sets of teeth-a labial vestigial set, then a functional set, and lingual to this a structure capable of producing one or more replacing sets. Further consideration of these sets will be found in m y general conclusions. The Molar Cusps l. Erinaceus in the adult condition has three molar teeth in each jaw, the first of these being large, while ^- are reduced. Ml. •> The first two upper molars are quinquetubercular, being provided with two well-developed external cones, the paracone and metacone, two internal ones, the protocone and the hypocone, together with a small central metaconule, this last being the most variable constituent. In addition there is a slight but complete cingulum. The lower molars (1 & 2) are also quinquetubercular, being modified trituberculo-sectorial teeth, in which the heel has In the descriptive portion of this paper Osborn's nomenclature of the molar cusps (13) is used, but I do not thereby imply that I believe in all cases the homology of the cones has been correctly interpreted ; in fact, in the general summary I endeavour to show that the cusp usually termed the protocone in the Insectivorous molars is not homologous in all the genera. |
