PISCES 363 



reduced; the ventral lobe develops until the whole structure 

 appears symmetrical again (homocercal) . Found in bony fishes 

 (Fig. 179, CD). 



Fig. 180. 



Pig. 180. Skull of Cod ipadus morrhua). From Nicholson, after Owen, b, branchiostegal 

 rays borne on C.A., the ceratohyal bone; d, dentary portion of the mandible;/, frontal; A.??*., hyoman- 

 dibular; i.o., interoperculum; I, lachrymal; m, maxilla; n, nasal; o, operculum; p.m.^ premaxilla; 

 P.O., preoperculum; p.s.^ parasphenoid; 2, quadrate; 5.0., sub-operculum; s.oc, supra-occipital. 



Questions on the figure. — ^What is the operculum? How many bones are 

 associated to form it? Which bones are figured as bearing teeth? Which of 

 these bones belong to the cranium proper ? What is the difference between cranium 

 and skull? What do authors believe to be the origin and homology of the chief 

 facial bones? 



381. Covering. — Most fishes are more or less covered by 

 scales or scutes developed in the dermis and lying between the 

 dermis and epidermis. The scales often receive a layer of 

 enamel from the epidermis. In form they may be cycloid 

 (round, with smooth margin), ctenoid (toothed margin), placoid 

 (plate-like bodies often bearing points covered with enamel), 

 and ganoid (thick rhomboid scales covered over with enamel, 

 and often closely articulated into a coat-of-armor) . The scales 

 are usually placed as shingles are on a roof, and doubtless pro- 

 tect the fish from mechanical injuries. A good many species 

 of fishes are destitute of scales altogether, the skin of such often 

 being supplied with numerous mucous glands. In many ex- 

 tinct forms the external covering was made up of large plates 

 fused into a dense armor. 



