Dr. Günther, in the British Museum Catalogue of Fishes, uses the generic title of _Ditrema_, which I have adopted. The first glance at the fish, as it lies on the table or on the beach, would lead you to pronounce it a _Pomotis_ (belonging to the family _Percidæ_): the northern _Pomotis_ (_P. vulgaris_) is a good example, and very common along the shores of Lake Huron, where I have often caught them. Or, on the other hand, you would be perhaps tempted to call it a _Sparus_; the gilthead (_S. auratus_) may be taken as a type suggesting the resemblance. This fish is taken in large numbers in the Mediterranean, and occasionally on the French and Spanish coasts. But a close investigation into the more marked generic and specific characters, apart from their reproducing organs, at once clearly shows they belong neither to the one family nor the other; they differ much more from the percoids than from the sparoids, but the cycloid scales remove them at once from the sparoids, in which the scales present a very uniform etenoid type. The illustration represents a female _Ditrema argenteum_, Brit. Mus. Cat., ‘Fishes.’ _Amphistichus argenteus_, Agass., Am. Journ., 1854; Soc. Nat. Hist., 1861, p. 131; Pacif. R. R. Exp., ‘Fishes,’ p. 201. _Mytilophagus fasciatus_ (Gibbons). _Amphistichus similes_ (Grd.). The middle dorsal spines are either nearly as long as, or somewhat longer, than the posterior; scales on the cheek, in five series, somewhat irregularly disposed. The height of the body is rather more than a fourth of the total length (without caudal); jaws equal anteriorly; the maxillary extends to below the centre of the orbit; lips thin, the fold of the lower interrupted in the middle. For description of species, _vide_ Appendix, vol. ii. CHAPTER V. STICKLEBACKS AND THEIR NESTS—THE BULLHEAD—THE ROCK-COD—THE CHIRUS—FLATFISH. The genus _Cottoidæ_, (fish having mailed cheeks) has a great many representatives, common on Vancouver Island and the British Columbian coasts. The least of the family, the stickleback, is so singularly different from most other fishes in its habits, as to merit the first consideration.