This section is from the book "American Game Fishes", by W. A. Perry. Also available from Amazon: American Game Fishes: Their Habits, Habitat, and Peculiarities; How, When, and Where to Angle for Them.
Body deep and compressed, rather elongate, with slender caudal peduncle when young; short, deep and almost orbicular in very old specimens; head moderate, about one-third the length, with short snout, large eye, and steep profile; depth of body about half the length, in old specimens somewhat more; mouth quite small, the maxillary not reaching eye; opercular flap large, entirely black, with narrow margin at base, nearly as broad as long in adults; in young specimens the flap is usually quite small, and broader than long; fins large; dorsal spines very high, often higher than soft rays in young, their length about equal to the distance from snout to posterior margin of eye; pectoral fins very long and falcate, reaching beyond beginning of anal; scales moderate; those on cheeks in about six rows; lateral line with 45 to 48. Coloration, adults dark olive or bluish green; belly and lower parts more or less coppery; no blue stripes on the cheek; a large dusky or "inky" spot on the last rays of dorsal and anal; young specimens show several undulating or chain-like transverse olive bars, and a bright purplish luster in life. Length 6 to 10 or 12 inches.
This species is the most widely diffused of all our Sunfishes, and westward it is everywhere the most abundant. Like Lcpomis megalotis it is subject to very great variations in form, coloration, and general appearance, yet it is usually, of all Sun-fishes, the species most readily recognized.
This fish, called the Blue Gill, in Michigan, is abundant in all waters from New York to Dakota, and thence southward to Florida and the Rio Grande. It reaches a larger size in the North, and in the vicinity of Lake Michigan it is the most important of the tribe. In large lakes it grows large, but in small streams it adapts its body to what it can find to eat - an arrangement not unknown elsewhere in the class of fishes.