In the type, the symphysis, as shown by Van Beneden and Gervais’ figure, plate 24, fig. 2_a_, is a little more than one-fifth the length of the mandible. The same relative proportion is found in the Atlantic City specimen, but, as the latter is a younger individual, one would expect the symphysis to be shorter. The figure of Van Beneden and Gervais gives the impression that in the type the end of the mandible is broken, and that, hence, the symphysis is shorter than it was originally. It will be observed that figures 2 and 2_a_ do not agree as regards the length between the tooth and the end of the jaw, figure 2_a_ showing a greater length. In figure 2, however, the jaw seems rather too long for the cranium, and if the greater length of the symphysis shown in figure 2_a_ were introduced, it would certainly be so. The explanation of this discrepancy is not readily found; but one may be allowed to think that the symphysis is not so blunt in the type as is shown in figure 2. In the Atlantic City specimen the superior lateral free margin of the symphysis is straight, while in the type it is much elevated. This is no doubt due to difference in age and possibly in sex. The type shows three or four mental foramina, while the Atlantic City specimen has one large posterior one and seven smaller ones anterior to it. Another peculiarity of the latter specimen is that the coronoid process is situated much in advance of the condyle, while the angle extends considerably behind it. In the type both are nearly in line with the condyle. I am unable to explain this difference. In the Atlantic City specimen the axis of the tooth where it emerges from the alveolus is 91 mm. from the end of the jaw. The portion of the tooth above the alveolus is 11 mm. long at the base and 12 mm. high. It is conical and sharp pointed, and is inclined forward and a little outward, especially at the tip. At the alveolus the transverse breadth of the tooth is 5 mm. The much larger tooth in the type indicates that that specimen was a male. The mandible of the Atlantic City specimen of _M. europæus_ differs from that of _M. bidens_ in the relative shortness of the symphysis, the large number of mental foramina, the more anterior position of the tooth, and the direction of the crown, which is forward instead of backward. _Dimensions of the type and two other skulls of Mesoplodon europæus._ Column headings: A: English Channel, type,[a] adult. B: North Long Branch, New Jersey, female, adult. C: Atlantic City, New Jersey, 23346 U.S.N.M., male, young.