This is the skull of a full-grown animal, and yet it is not so large as the skull of a newly born specimen of _Orca_. Mr. Flower, judging from the figure, believed it to be the skull of a very young animal; but on examining the skull along with me he became satisfied, from the solidity and definite form of the bones, that it is the skull of a full-grown though not aged specimen. 10. LEUCOPLEURUS. Leucopleurus, _Gray_, _P. Z. S._ 1866, p. 216; _Synops. Whales & Dolph._ p. 7. Beak of the skull rather flat above and elongate, bent up on the edge in front of the notch, narrow behind, as long as, or slightly longer than, the length of the brain-case. Teeth-line reaching nearly to the notch. Teeth small, five in an inch. First and second cervical vertebræ united by their bodies, third and fourth by the spinous processes. Vertebræ 81:—C. 7. D. 15. L. and C. 59. 1. Leucopleurus arcticus. B.M. Leucopleurus arcticus, _Gray_, _Synops. Whales & Dolph._ p. 7, t. 6. f. 3-5 (fœtus), t. 12 (skull), t. 26. f. 3 (tongue). Lagenorhynchus leucopleurus, _Gray_, _Cat. S. & W._ p. 273. no. 9; _Gervais, Ostéogr. Cét._ t. 36. fig. 4. Beak of the skull twice as long as the width at the notch. Teeth small, five in an inch. Inhab. North Sea. 11. LAGENORHYNCHUS. Lagenorhynchus, _Gray_, _P. Z. S._ 1866, p. 216; _Cat. S. & W._ p. 272; _Synops. Whales &. Dolph._ p. 7. Beak of the skull rather flat above, bent up on the edges in front of the notch, deep, broad behind, rather shorter than the length of brain-case. Teeth-line reaching nearly to the notch, large, three in an inch. First and second cervical vertebræ united by their bodies; the third, fourth, fifth, sixth, and seventh free. Vertebræ 88:—C. 7. D. 14. L. and C. 67. 1. Lagenorhynchus albirostris. B.M. Lagenorhynchus albirostris, _Gray_, _Cat. S. & W._ p. 272. no. 8; _Synops. Whales & Dolph._ p. 8, t. 10. f. 2, t. 11 (skull); _Gervais, Ostéogr. Cét._ tab. 36. fig. 5. The beak of the skull once and one-half as long as the width at the notch. Inhab. North Sea, Yarmouth. Tribe IV. _PSEUDORCAINA._ Head rounded in front, very convex, not beaked. Teeth conical. Beak of the skull depressed, broad, scarcely so long as the brain-cavity. 12. PSEUDORCA. Pseudorca, _Gray_, _Synops. Whales & Dolph._ p. 8. Head rounded, convex; body moderate; dorsal fin moderate, in the centre of the back; arm-bones very short and thick, the humerus rather the shortest.