| OCR Text |
Show 564 MR. M. E. WOODWARD ON [May 5, attained equal importance with the trigon and developed two large cusps, an internal entoconid and an external hypoconid This tooth is further specialized in the partial suppression of the paraconid, the antero-internal cone of the trigon, while the pro-toconid (antero-external) and the metaconid (postero-internal) are very large. Taking the case of the upper molar first, in both m y specimens m-l was fairly advanced, showing indications of four cones viz. the para-, meta-, proto-, and hypo-cones, here mentioned in order of size ; the hypo-cone was obviously the last of the four to develop as it is only just recognizable in the younger specimen; in the older stage these cones are larger, and the first trace of the meta-conule is here apparent. Thus in m j . W e can say for certain that the metaconule appears last and the hypocone next to last, but the trigon was too advanced in m y specimens to determine the developmental order of its cones. But where mj. fails us m. 2 comes to our rescue: in the younger stage two cones only were apparent, viz., the two external ones, the para- and metacones, and of these the former was much the largest and must obviouslv have developed first; immediately internal to this was a low shelf, in the position of the future protocone, but at present no cone was recognizable. In the older stage of this tooth the protocone has appeared in this position, and a faint indication of the heel is visible. W e may thus state with certainty that the order of cusp ontogeny is as follows :- 1. Paracone 1 2. Metacone > Trigon. 3. Protocone J 4. Hypocone. 5. Metaconule. In the lower jaw, as in the upper, the first molar was too far advanced, all five cones being recognizable, but nevertheless differing greatly in their relative sizes ; the following is their order according to size, viz., proto-, meta-, ento-, hypo-, and paraconid, the last being only just recognizable. An examination of m-_2 throws more light on the subject, for here only three cones are developed as yet, viz., the proto-, meta-, and entoconid, a slight antero-internal extension of the tooth-germ indicating the position of the future paraconid, while a similar but larger postero-external platform marks the hypoconid. The protoconid is larger than the metaconid at this stage, and the metaconid than the entoconid, the probable order of development being:- 1. Protoconid. 2. Metaconid. 3. Entoconid. 4. Hypoconid. 5. Paraconid. The ordinal position of the paraconid in the ontogeny may |
