Pectoral skin with longitudinal folds : margin of the under lip semicircular.
Balsenoptera Rorqual, Scoresb. Arct. Reg. vol. 1. p. 482. B. Mus-culus, Flem. Brit. An. p. 30. Round-lipped Whale, Penn. Brit. Zool. vol. in. p. 58.
Entire length from seventy to eighty-six feet.
Strongly resembling the last species, but said to grow to a much larger size: differs also in having the lower lip much broader than the upper, and semicircularly turned at its extremity, while the upper is somewhat sharp or pointed at the tip: gape large; longest lamina of whalebone three feet in length.
Found in the Scotch seas with the preceding. Said to feed principally on herrings.
The characters of the species in this sub-genus are not sufficiently well understood to admit of being laid down with any degree of certainty, and there can be little doubt that the species themselves have been unnecessarily multiplied. Cuvier appears to be of opinion (Regne An. torn. i. p. 298). that even the B. Physalus may eventually turn out to be the same with the B. Boops, and that the supposed difference between them may be the result of hasty and imperfect examination. There is likewise but little dissimilarity between the B. Boops and the B. Mus-culus, and it is not as yet satisfactorily ascertained that even that little is to be depended upon as constant in all cases. With respect to the Balcena rostrata of Fabricius and Hunter (Phil. Trans. 1787. pi. 20). which occurs not unfrequently on the British coasts, it is probably nothing more than the young state of one of the two last mentioned; in the opinion of Cuvier*, of the B. Boops.
* See his Ossemens Fossiles (torn. v. p. 365). which contains a valuable dissertation on the different species of the Cetacea.