The Conchological Society of Great Britain and Ireland

Varies greatly in form and colour.
Body (excluding appendages)
Frequently 25mm – 60mm long , but up to 90mm, or, exceptionally, 120mm. Flesh mostly opaque; white, brown, grey, yellowish, ginger, rosaceous or pinkish, powdered and freckled with opaque white and gray, brown or lilac. White anal papilla usually concealed among cerata.
Cerata
Numerous, arranged in about 20 close rows of 12 – 24 cerata each, on each side of the body, concealing sides, but front half, at least, of dorsum usually exposed. Ceras shaped like willow leaf, rather flattened and swept back becoming longer and attenuated when active. Anterior cerata small, positioned well in front of rhinophores. Dense pigment, variously coloured, conceals internal yellow digestive gland, except for small translucent area at base of posterior face (only exposed when ceras swept forwards). Tips whitish.
Rhinophores
Smooth, short, straight, conical with trucated apex, and contractile. Opaque, similar colour to body, or darker shade, darkening more when contracted. Apex pale; yellowish or whitish.
Head
Anterior a broad smooth curve between oral tentacles, unless drawn back and puckered. Distance between oral tentacles about 3 times thickness of tentacle base. Oral tentacles usually a little longer than rhinophores. Diagnostic pale triangle in front of  rhinophores on some, but not all specimens. Its vertices often extend as lines between rhinophores and onto  oral tentacles. Oral tentacles usually distally powdered white and proximally flecked grey when no lines on them. Internal eyes at base of rhinophores not, or barely, visible through opaque pigment.
Foot
Rather broad, especially at front. Anterior gently convex with deep marginal  groove and small distinct propodial tentacles. Sole translucent white, revealing rose tinted ovotestis.