on the coast of Nantucket by Messrs. H. M. and S. C. Martin, of Roxbury. It belonged to the genus Mesoplodon, as characterized by Gervais, and ought to be separated from the fossil Ziphius, described by Cuvier. Professor Agassiz, however, questioned whether Mesoplodon was not identical with Delphinorhynchus, previously described by De Blainville. The specimen found at Nantucket measured 16 feet in length.[6]

SKULL.

The skull of this Nantucket specimen, which I have before me, is thoroughly adult. That the specimen is a female is probable from the fact that the teeth (one of which is preserved), though fully developed, are only two-thirds as broad and three-fourths as long as those of Sowerby’s specimen (the type of the species), which was an adult male.[7] The skull is 765 mm. long, and about 30 mm. are lacking from the end of the beak, so that the original length was about 795 mm. It appears to be, therefore, rather the largest skull of the species of which there is any record. The specimen itself, according to Dr. J. A. Allen, was 16 feet 3 inches long.[8] The largest European skull appears to be the one in the Edinburgh Museum, described by Sir William Turner in 1872.[9] The length of this is 749 mm. The specimen was a female, but though the skull is so large, the mesirostral cartilage was not ossified, and the individual was, therefore, probably not thoroughly adult. Two other European specimens, of which the total length was almost identical with that of the Nantucket specimen, were (1) the adult female obtained at Overstrand, England, in 1892, and recorded by Southwell and Harmer[10] (length 16 feet 2 inches, straight); (2) the adult male obtained at Brodie House, Scotland, in 1800, and recorded by Sowerby[11] (length 16 feet). The length of the skull is not given for either of these specimens. The adult male obtained at Rugsund, Norway, in 1901, and recorded by Grieg,[12] was only 15 feet 1 inch long, but some of the measurements of the skull are as large as, or even a little larger than, those of the Nantucket skull. The total length of the skull was not given, as the end of the beak was lacking.

Grieg’s figures of the Rugsund skull afford a very satisfactory basis for comparisons between that specimen and the Nantucket skull (Pl. 1, fig. 1). Both skulls show the comparatively narrow frontal region, the moderately developed tubercle anterior to the anteorbital notch, and the low maxillary ridge, which are characteristic of the species. In both skulls the anterior prolongation of the ethmoid is lanceolate and flat, but in the Rugsund skull the apex is truncated. In the latter also the posterior end of the mesirostral ossification is divided into three longitudinal sections by two lateral and somewhat divergent grooves, while in the Nantucket skull there is only a single median groove. These differences may safely be regarded as individual. Toward the distal end the surface of the ossification in the Nantucket is pitted and irregular and descends much below the level of the premaxillæ. It ends distally at the same point with the vomer. In this skull the proximal end of the premaxillæ and adjoining plate of the maxillæ are somewhat less reflexed than in the Rugsund skull. The shape of the superior margin of the supraoccipital is alike in both.

There are no well-defined differences in the relative thickness of the beak at the base or in the form and position of the visible portion of the palatines, but in the Nantucket skull the mass of the combined frontal and lachrymal anterior to the orbit is less rounded and more triangular than in the Rugsund skull. The temporal fossæ also have a postero-superior angular enlargement not seen in the latter.

In the Nantucket skull the rostral portion of the premaxillæ is high and at the distal end vertical. The superior profile is somewhat convex, and the superior free margin rounded proximally, but sharp distally. The least distance between the free margins is 10 mm.

The pterygoids are cut off from the maxillæ anteriorly by a very narrow band of the palatine, which connects with a broad band externally and a lanceolate segment internally. The inferior pterygoid ridges diverge anteriorly. The broad surface internal to them is concave. The external border of the pterygoid sinus is nearly straight. An elongated, fusiform section of the vomer is visible on the inferior surface of the beak at the middle for a distance of 158 mm., and a small lozenge-shaped section, ill defined, is visible between the pterygoids and palatines. (Pl. 4, fig. 1.)

The expanded anterior end of the malar is rhomboidal in form, with an external free margin 11 mm. long. Anteriorly it does not form part of the margin of the anteorbital notch.

The lachrymal is irregularly oblong, with an external free margin 35 mm. long and 12 mm. thick. The distance from the anteorbital notch to the anterior end of the orbit is 60 mm. (Pl. 7, fig. 1.)

The lateral free margins of the basioccipital are extended posteriorly beyond the exoccipitals, which is a character indicative of age.

The supraoccipital has a distinct median ridge, with a longitudinal depression on each side, bounded externally by a prominent convexity. (Pl. 10, fig. 1.)

MANDIBLE.

The mandible is slender, with a very elongate symphysis, which measures 237 mm. The inferior outline of the ramus is strongly concave at the middle and slightly convex posteriorly, while the symphysial portion is bent upward. The superior outline is concave both behind and before the tooth, and also immediately anterior to the coronoid process. At about the beginning of the posterior fourth the outline is convex, and the mandible at this point is nearly as deep as at the coronoid process. The superior surface of the symphysis slopes down on each side to the median line, but each half of the surface is itself nearly plane. (Pl. 11, figs. 1, 2, and 5.)

The alveolar groove anterior to the tooth is very distinct throughout and is without septa and open at the bottom. It ends