Order:	Scorpaeniformes
Family:	Cottidae
Genus and species:	Cottus gobio

William Yarrell (1836) in "A History of British Fishes":

THE RIVER BULLHEAD, MILLER'S THUMB,

TOMMY LOGGE.

Cottus	gobio,	LINNÆUS. BLOCH, pt. ii. pl.39.
"	"	CUV. et VALENC. Hist. Nat. des Poiss. t. iv. p. 145.
"	"	River Bullhead,	PENN. Brit. Zool. vol. iii. p. 29 1, pl. 43.
"	"	"	DON. Brit. Fish. pl. 80.
"	"	"	FLEM. Brit. An. p. 216, sp. 157.

Generic Characters. - Head large, depressed ; teeth in both jaws and in front of the vomer, small, sharp, none on the palatine bones ; preoperculum or operculum armed with spines, sometimes both ; branchiostegous rays 6 ; gill-openings large ; body attenuated, naked, without scales ; two dorsal fins, distinct or very slightly connected ; ventral fins small.

THE RIVER BULLHEAD is an inhabitant of almost all the fresh-water streams of the whole of Europe, from Italy to Sweden ; and most of the streams in this country that in their course run over sand or gravel produce this fish. It occurs also in the north of Ireland, in Belfast and Londonderry. Its length seldom exceeds four or five inches, and it is generally found among loose stones, under which, from the peculiarly flattened form of its head, it is enabled to thrust itself, and thus to find a hiding-place. When disturbed, it swims rapidly. The term Bullhead has been attached to all the species of the genus Cottus, on account of the large size of the head ; as we also use the words Bullfinch, Bullfrog, Bulltrout, and Bullrush, to indicate species of large comparative size.
As the term Bullhead is thus considered to refer to the large size of the head, so the name of Miller's Thumb given to this species, it has been said, is suggested by, and intended to have reference to, the particular form of the same part.
The head of the fish, it will be observed by the accompanying vignette, is smooth, broad and rounded, and is said to resemble exactly the form of the thumb of a miller, as produced by a peculiar and constant action of the muscles in the exercise of a particular and most important part of his occupation.
It is well known that all the science and tact of a miller is directed so to regulate the machinery of his mill, that the meal produced shall be of the most valuable description that the operation of grinding will permit when performed under the most advantageous circumstances. His profit or his loss, even his fortune or his ruin, depend upon the exact adjustment of all the various parts of the machinery in operation. The miller's ear is constantly directed to the note made by the running-stone in its circular course over the bedstone, the exact parallelism of their two surfaces, indicated by a particular sound, being a matter of the first consequence: and his hand is as constantly placed under the meal-spout, to ascertain by actual contact the character and qualities of the meal produced. The thumb by a particular movement spreads the sample over the fingers ; the thumb is the gauge of the value of the produce, and hence has arisen the sayings of, "Worth a miller's thumb;" and, "An honest miller hath a golden thumb;"* in reference to the amount of the profit that is the reward of his skill. By this incessant action of the miller's thumb, a peculiarity in its form is produced which is said to resemble exactly the shape of the head of the fish constantly found in the mill-stream, and has obtained for it the name of the Miller's Thumb which occurs in the comedy of "Wit at several Weapons," by Beaumont and Fletcher, act v. scene i. ; and also in Merrett's "Pinax.''
Although the improved machinery of the present time has diminished the necessity for the miller's skill in the mechanical department, the thumb is still constantly resorted to as the best test for the quality of flour.
This version of the cause of the application of the term Miller's Thumb to our River Bullhead, was communicated to me by John Constable, Esq. R.A. ; whose father, being one of those considerable millers with which the counties of Essex and Suffolk abound, was early initiated in all the mys-teries of that peculiar business. He also very kindly lent me a view of an undershot water-mill at Gillingham, worked by a branch of the stream from Stourhead, which is represented in the vignette.

D 6 to 9 - 17 or 18 : P. 15 : V. 3 : A. 13 : C 11.

The size and form of the head has been already noticed : the mouth is wide, jaws nearly equal, numerous small sharp teeth in both jaws and on the anterior part of the vomer ; no spines on the head ; irides yellow, pupils dark blue ; preoperculum. with one spine curved upwards ; the operculum. ending in a flattened point; the dorsal fins united by a membrane ; rays of all the fins prettily spotted ; general colour of the body above dark brownish black, sides lighter, with small black spots ; under surface of the head and belly white ; the vent in a vertical line tinder the commencement of the second dorsal fin. This species spawns in summer.

* Ray's "Proverbs."

Frank Buckland (1880) in "Natural History of British Fishes":

BULLHEAD.

(Cottus gobio)

Local names: Miller's Thumb, Bullhead, River Bullhead, Tommy Logge, Tom Culls. French: Chabot commune. German: Der Kaulkopf, Die Muhlkoppe Rotzkolbe, Groeppe, Gruppe, Koppe, Der Krautzenkopf.

THIS little fish is found in rivulets and brooks about old timber bridges, where they look like mice running up and down close to the woodwork. He has an enormous head and mouth and a very sharp-pointed body. On each side of his head there is a very sharp needle-pointed spine. The iris of the eye is very brilliant, reminding us of a toad's eye. The bullhead may be said to be the chameleon among fishes; it is very rare to meet many of the same colour. The colours they assume are yellow, brown, orange, emerald green, &c. Nobody knows when or how they spawn.
They are very common in the Itchen, in Hampshire. I was educated at Winchester College, and one of the great sports of myself and other boys was to "spear Tom Culls." Our mode of proceeding was to fasten an ordinary dinner fork on the end of a stick, and "spear the Tom Culls" as they lay among the stones. This required great dexterity, as the fish was very slippery, and the fork, not being barbed, was a difficult instrument to catch them with. Tom Culls are said to be great enemies to trout eggs, and they would doubtless eat the fry out of the gravel if they had the chance.
By the kindness of Lord Radnor, Longford Castle, Salisbury, I received in April, 1880, a grebe (Podiceps minor), choked by a bullhead. The fish was fixed so firmly in the bird's mouth that I found it would go neither backwards nor forward, so I could neither press it down the oesophagus nor pull it out altogether. Mr. Grebe evidently was not aware that the miller's thumb was armed with two very sharp spikes on each side of the gill cover, and when the fish found himself in trouble he simply expanded these spines, which fixed him so firmly in the bird's mouth that he died from suffocation. I have had two or three specimens sent me of kingfishers destroyed by bullheads sticking in their throats.

Alwyne Wheeler (1969) in "The Fishes of the British Isles and North West Europe":

Miller's Thumb, Bullhead

Cottus gobio (Linnaeus, 1758)

NAMES Fr. Le Chabot; Du. Rivier donderpad; Ge. Kaulkopf; Da. Flodulk.

IDENTIFICATION A small broad headed, rather flattened fish living under stones or in dense vegetation, in rivers and large lakes. A single, upwardly pointed spine on the pre-operculum. No scales on body. Lateral line canal extends to the base of the tail fin but canal pores often reach only to the middle of the second dorsal. Pelvic fins light in colour, not barred; the inner ray is equal to or slightly less than the outer ray; the fins only just reach to the vent, and are often shorter. The colour is very variable, usually dark brown on the back, heavily mottled, sometimes with distinct bars running across the sides; all the fins except the pelvics are dark with irregular light fines. Grows to 4 in (10 cm).
ID. V-IX; 2D. 13-19; A. 10-15; lateral line 30-5.

BIOLOGY Common on hard bottoms in streams, rivers and lakes where the bottom is stony and where the water flow is moderate to weak, it is probably not found in water deeper than 5 fathoms (9 m). It lives under stones and other obstructions, solitary except in the breeding season, and rarely emerges except at night or during dull weather. Breeding occurs from March to May (May further north) in a slight hole excavated under a stone by the male. The eggs are laid on the underside of the stone, and are guarded by the male until hatched in about four weeks. The fry scatter from the nest on hatching and take shelter under stones.
Growth in the first year is to a maximum of 1 ¾ in (4.5 cm), in the second to 2 ½ in (6 cm). Most bullheads Of 2 ¾ in (7 cm) are four years old, but exceptionally fast growing males (which are larger than the females generally) may reach 3 ½ in (9 cm) in five years. They feed mainly on bottom-living invertebrates, chiefly crustaceans, Gammarus and crayfish, but they also eat numerous types of insects, particularly ephemeropteran nymphs, plecopteran and trichopteran larvae. Although the bullhead lives in places selected by salmonoid fishes for spawning redds, their eggs and alevins only occasionally figure in its diet.

DISTRIBUTION