■ADIPOSE LIDS' 



383 



aperture between well-developed lids is a narrow vertical ellipse (Fig. 

 13 2d), but tends toward a large circle as the lids are reduced in extent. 

 Where the lids are narrow, they are usually continuous inferiorly as well 

 as superiorly, instead of being overlapped (Fig. 132e, g). There are in- 

 stances in which a given genus has prominent vertical lids, while a related 

 one with closely similar habits is without them, perhaps owing to total 

 disappearance. Tarpon for example has no lids, whereas the ten-pound- 

 ers (Elops) have them well developed. 



Lids of the types just described are especially characteristic of the 

 herrings and their allies, constituting the 'clupeoids', among the soft- 

 rayed teleosts (Malacopterygii) . They have been independently evolved 



Fig. 132 — 'Adipose' lids in various teleost fishes (drawn from preserved specimens). 



a, Salmo gairdnerii irideus. b, Clupea harengus. c, Nematalosa nasus. d, Pomolobus 

 chrysochloris. e, Hiodon tergisus. f, Rastrelliger loo. g, Mugil cephalus. 



/- fold or ridge in head skin; /n- 'false nirtitating membrane'; Mimbus corneae; m- margin of 

 drcumocular sulcus; o- wall of orbit; p- pupil; s- extremity of recess under m. 



also by equally swift, pelagic members of the more advanced spiny-rayed 

 division ( Acanthopterygii) . The correspondence between Pomolobus 

 (Fig. 132d), a clupeoid, and Rastrelliger (Fig. 132f), a scombroid, is 

 quite perfect. Mugil cephalus, another acanthopterygian (Fig. 132g), 

 has its counterpart in the clupeoids /I m/)/?/Won and HjoJon (Fig. 132e). 

 Among both malacopterygians and acanthopterygians there are families 

 in which the aperture between the vertical lids has been quite obliterated, 

 so that there is an unbroken covering over the eye. Though this is of 

 course also a streamlining adaptation, and probably an even better one 

 than the separate, apertured lids, it is discussed later in connection with 

 the other types of 'spectacles' to which it is morphologically related. 



