362 ZOOLOGY 



reduced; the ventral lobe develops until the whole structure 

 appears symmetrical again (homocercal) . Found in bony fishes 

 (Fig. 179, C, D). 



FIG. 180. Skull of Cod (Gadus morrhua). From Nicholson, after Owen, b, branchiostegal 

 rays borne on c.h., the ceratohyal bone; d, dentary portion of the mandible;/, frontal; h.m., hyoman- 

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

 P.O., preoperculum; p.s., parasphenoid; q, quadrate; s.o., 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 of bony matter 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 doubt- 

 less protect 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 



