OCR Text |
Show 1896.] MAMMALIAN DENTITION. 567 ppm.2, and fig. 14, PP,"^.3). The upper dpm. 2 is larger than dpm. possibly at times it cuts the gum ; it is well seen in a specimen in the British Museum (fig. 13, dpm. 2), probably the one Thomas described; no trace of dpm.2 is seen in that specimen, andit«is probable that this tooth is either shed in utero or absorbed. -,t are quite normal, the milk representative being large, and the lingual growths of the dental lamina, which give origin to the enamel-organs of their successors, being conspicuously swollen (fig. 14, ppm. 3). This tooth is somewhat similar in the two dentitions, but distinctly larger in the adult. -1-- -.-These are the largest and most complex of the pre- Dpm. 4 ° J L molars, and both exhibit conspicuous lingual specializations of the dental lamina, the enamel-germs of their successors. These germs are developed in front of the deciduous teeth, and although the lingually-placed dental lamina is continued back by the side of dpm. 4, it is no longer swollen to form an enamel-germ. The Molars. In the stage examined two molars were present, above and below, but save in the case of ni. 1 no labial or lingual developments of the denial lamina were to be seen. M. 1, however, exhibited both a lingual and a slight labial growth, similar to those seen in Erinaceus. The Cusps. The molar teeth of Gymnura resemble those of Erinaceus in pattern; like that genus they exhibit five cusps, wdiich are strongly developed, and in the upper jaw- a well-marked cingulum, with a small anterior and posterior cusp, is present in addition; in the lower jaw the paraconid is less developed than in Erinaceus. My fostal specimen was rather old for an exact determination of the cusp ontogeny, most of the cusps being well-formed. In m. 1 all five cusps were present, and had attained nearly their full development; the following is their order in size: proto-, meta-, para-, and hypocones, the smallest being the metaconule. M. 2 was less developed, and here the para- and metacones were the most strongly developed, while the protocone was present in the form of a large autero-external shelf, but hardly as yet developed into a distinct cusp, though the hypocone and metaconule had done so. Probable order of cusp-development:- f. Paracone. 2. Metacone. 3. Protocone as a shelf. 4. Hypocone. 5. Metaconule. 6. Protocone as a cusp. |