INTRODUCTION xvii 



compressed, deepest at or in advance of the middle 

 of the length and tapering posteriorly ; sometimes it 

 is deep and strongly compressed, as in the stately 

 Bream which lives in still waters, sometimes long and 

 cylindrical, as in the Eel, which wriggles through the 

 ooze and in and out of holes and crevices. Three 

 regions may be distinguished — head, trunk, and tail, 

 the gill-opening marking the limit between head and 

 body, the vent that between trunk and tail. 



The head may be naked or scaly, and the bones 

 may be exposed or may be concealed beneath a thick 

 skin ; the eyes are typically lateral, and in front of 

 them appear the nostrils, two on each side ; the snout 

 is the region in front of the vertical through the 

 anterior edge of the eye, and the postorbital part of the 

 head that behind the vertical through the posterior edge 

 of the eye, whilst the interorbital region is that part of 

 the top of the head which lies between the bony orbits. 



The eye may be surrounded by a complete series 

 of circumorbital bones, but the supra-orbital is absent 

 except in a few generalized types, whilst the prse- 

 orbital is usually the largest, and the rest, the sub- 

 orbitals, vary greatly in their development ; the region 

 behind and below the eye is termed the cheek, and is 

 bounded posteriorly by the prae-operculum, a bone 

 which is sometimes crescentic, but more often angular, 

 with a vertical and a horizontal limb ; the posterior 

 edge of the prae-operculum may be entire, serrated, 

 or spinate. Behind the prae-operculum appear the 

 movable opercular bones or gill-covers, comprising 

 the operculum above, the suboperculum below and 

 behind, and the interoperculum below and in front ; 

 below these the gill-membrane is supported by a 

 series of branchiostegal rays, very variable in number. 



