PISCES. 



359 



3. The vertebral column may become still more bent and 

 reduced; the ventral lobe develops until the whole structure 

 appears symmetrical again (homocercal). Found in bony- 

 fishes (Fig. 176, C, D}. 



FIG. 177- 



FIG. 177. 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; f, frontal; A.m., hyomandibular; i.o., interoperculum; /, lachrymal; m, 

 maxilla; n, nasal; o, operculum; p.m., premaxilla; P.O., preoperculum; p.s., parasphe- 

 noid; 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 differ- 

 ence between cranium and skull? What do authors believe to be the 

 origin and homology of the chief facial bones? 



374. 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). A good many species of fishes are destitute 



