| OCR Text |
Show 1892.] NUMERICAL VARIATION IN TEETH. 107 is also divided by the grooves so as to form two small cusps. Each of these teeth is therefore an imperfectly double structure, and may be described as being just halfway between a single tooth and two teeth. On the left side in the upper series, as the vis-a-vis to one of these double teeth, there are two complete teeth, standing near Fig. 1. Ommatophoca rossii, having the first upper premolar on the right side bigemi-nous, and on the left side represented by two complete and similar teeth. In the lower series the first premolar was bigeminous on both sides. From a specimen in the British Museum. together, but having separate sockets divided from each other by a bridge of bone. The dental formula for this skull taken as it stands is :-i. g^, c. j-:, p. + m. ^5, for since the bigeminous teeth are not completely divided into two, they must be reckoned as single teeth. Phoca grcenlandica.-A specimen is preserved in the Leyden M u seum having the arrangement shown in the figure (fig. 21, p. 108). The dentition of the lower jaw is the same on both sides. In the upper jaw there are on the left side six teeth behind the canine, while on the right side there are five, the normal number. But upon examination, one of these teeth, namely p* of the usual nomenclature, is an abnormally large tooth, being especially thick in a transverse direction, and has besides a deep cleft in the crown, 1 This figure was kindly drawn for m e by Mr. J. J. Lister. |
