66 MORPHOLOGICAL TYPES 



epidermis. The scales are thin, flat plates of material akin to 

 bone. They are kept throughout life, and enlarge by con- 

 centric additions. These scales have nothing to do with the 

 denticles or placoid " scales " of the dogfish. 



Fin-rays. — The fins are supported by fin-rays, but these, 

 instead of being horny and unjointed like the ceratotrichia of 

 the dogfish, are bony and jointed, and are called lepidotrichia. 

 In the more highly-developed bony fish like Gadus, the 

 lepidotrichia correspond in number to the radials of the axial 

 skeleton, in the dorsal and ventral median fins. There is a 

 lepidotrichium on each side of the tip of each radial, and a 

 joint between them enables the web of the fin to be raised or 

 lowered. At the edge of the fin, between the lepidotrichia, 

 there are some small unjointed horny rays called actinotrichia. 

 These correspond to the ceratotrichia of the dogfish. 



Skeleton.- — The cartilaginous skeleton corresponding to 

 that of the dogfish is present in early stages of development in 

 the bony fish. In the adult, most of this cartilage is replaced 

 by an altogether different skeletal material, viz. bone. Bones 

 which arise in this manner, i.e. replacing pre-existent cartilage , 

 are called cartilage-bones or replacing bones. Some bones, on 

 the other hand, have no cartilaginous precursor at all. These 

 arise independently, as more or less flat plates in relation to 

 the surface of the body, though they may sink deeper. These 

 are dermal or membrane-bones. There is no difference in 

 structure between cartilage-bones and membrane-bones, the 

 distinction applies only to the method of origin. Sometimes 

 a bone which develops as a cartilage-bone in one animal may 

 arise as a membrane-bone in another, and vice versa, though 

 these cases are rare. It is to be noted that as a rule a cartilage- 

 bone represents an ossification in a cartilaginous structure 

 which exists in the dogfish, whereas a membrane-bone is a 

 structure which is wholly unrepresented in the dogfish. There 

 is no doubt that the scales, fin-rays, and bones are kindred 

 structures. 



Skull. — As in other forms, the skull can be divided into the 

 neurocranium or brain-case, and the splanchnocranium or 

 jaws. 



