Length of mandible [e]1,245 1,236 ... 1,334 1,289 1,282 [f]883 1,360 Length of symphysis 310 294 ... 295 295 270 [f]145 310 Height at coronoid 211 222 ... 271 230 223 175 245 Distance from tip 34 45 ... 50 48 35 [f]22 60 of jaw to center of first tooth Distance from tip 155 159 ... 200 182 165 [f]87 195 of jaw to center of second tooth [a] From Van Beneden and Gervais figure. [b] A little broken at tip. [c] “Suprafrontal processes of maxillæ.” [d] Same point. [e] “Length of ramus.” [f] About 27 mm. lacking from tip of mandible. The foregoing measurements indicate a considerable variation in proportions among the different individuals, but there appears to be nothing that can be fixed upon in this small series to distinguish the two species by dimensions alone. EARBONES. The tympanic and periotic bones of _B. bairdii_ (Pls. 34-37) present a number of characters by which they may be distinguished from those of _B. arnuxii_. While of about the same size in both species, the two bones when in the natural position, viewed from without, are nearly square rather than triangular in outline in _B. bairdii_, the superior border of the periotic being nearly parallel with the inferior border of the tympanic, and the anterior lobe of the periotic being turned down nearly at right angles with the rest of the bone. The periotic is shorter anteriorly than the tympanic in _B. bairdii_, while the reverse is true in _B. arnuxii_. In the former species the eustachian canal of the tympanic is wider, the distance between the outer and inner lips being greater. The involuted portion of the inner lip is shorter and differently shaped. The groove between the postero-inferior lobes is wider. The periotic beside having a much shorter anterior lobe than in _B. arnuxii_ has also a smaller and smoother middle lobe, and the internal auditory meatus is smaller and more oblique. The dimensions of the bones in the Centerville beach skull, No. 49725, are as follows: _Tympanic_: greatest length, 62 mm.; greatest breadth, 46; least breadth of eustachian canal, 17; height at sigmoid process, 47. _Periotic:_ greatest length, 66; greatest breadth, 40; height at center of middle lobe, 35; length from tip of anterior lobe to anterior margin of internal meatus, 38. TEETH.