The very high second dorsal (about 22 rays) and anal fins (about 20 rays) of the lookdown, and their peculiar falcate outline with the second ray much the longest and the next 4 or 5 rays successively shorter make distinction easy between it and the moonfish. And its peculiar form is hardly less characteristic, for it shares with the moonfish a deep, rhomboid, but very thin flat body (the trunk is only about one and one-quarter times as long as deep), abruptly truncate in front, with slightly concave upper anterior profile, and tapering rearward to a slender caudal peduncle. The mouth is set so low and the eye so high that the expression of its face is very characteristic. When adult the first dorsal is reduced to 7 or 8 short inconspicuous spines, only the first 3 of which are connected by a membrane, and the ventrals are very small; but some of the spines of the first dorsal are very long in fry up to 4 or 5 inches in length, the ventrals are much longer than in the adults, and the anal fin is preceded by two short detached spines that disappear with growth. The caudal fin is deeply forked like that of other pompanos, and the pectorals are sharp pointed and falciform, reaching back behind the middle of the second dorsal fin.
Small specimens, and northern strays usually are small, are silvery above as well as [page 380] below, with the ground tint of the back leaden; the sides are barred with several crossbands, variously described as dark or golden. But these bands fade out with growth.
Reaches a weight of about 2 pounds.
Warm waters on the east and west coasts of America, north rarely to Cape Cod, straying to the Gulf of Maine and to Nova Scotia; common from Chesapeake Bay southward.
There were only three records for the lookdown in our Gulf up to 1933; two of them for Casco Bay, the third for Boston Harbor (Dorchester). But many small ones were reported from the traps at the mouth of Casco Bay during that autumn, one from Beverly on the north shore of Massachusetts Bay, and one from North Truro on Cape Cod. Evidently this was an unusual incursion, for no one would be apt to overlook so bizarre a straggler from the south.
 Jones (Proc. and Trans. Nova Scotian Inst. Sci., vol. 5, App., 1879, p. 89) and Honeyman (Trans. Nova Scotian Inst. Sci., vol. 6, 1886, p. 328) report young fry as occasionally found in the shore waters of Nova Scotia, presumably along the outer coast, for tropical fishes are taken oftener there than along the Gulf of Maine shore of the Province.