THE INTEGUMENT AND THE EXOSKELETON 83 



cusp or point of the placoid type. In the sturgeon the scales 

 consolidate into large, bony shields or scutes, and this principle 

 was carried to the extreme in the long extinct and nearly re- 

 lated groups of placddernis, where the entire fish was covered 

 with a heavy suit of mail, probably as a protection from the 

 huge molluscan forms which then thronged the seas. In the 

 sturgeon, however, these plates are not continuous, but are 

 arranged in longitudinal rows along the back and sides, leav- 

 ing large areas unprotected. In all ganoids similar scutes 

 cover the entire head, and fit together by their edges, forming 

 sutures, but leaving no appreciable intervals. These are fairly 

 definite in number and arrangement in the different species 

 and form the so-called dermal bones of the skull. [Cf. Chap. 

 V.] Certain of these, as the f rentals, parietals, maxillaries 

 and squamosals, persist in the highest groups ; others, like the 

 opercular and rostral series, disappear completely, while of an 

 extensive orbital series one alone persists as the lacrimal. 

 The dermal bones that line the mouth cavity, such as the 

 vomers, palatines and parabasal, retain the indications of their 

 origin longer than do the others, since, in many cases among 

 both fishes and amphibians, they are covered with teeth which 

 are arranged in imbricated series over a considerable area; 

 occasionally, even, as in the vomers of the frog larva, these 

 elements begin as separate conical teeth which fuse secondarily 

 to form the plate, thus repeating ontogenetically their mode of 

 origin. Nearly always, however, the history is curtailed, and 

 the dermal bones first appear as thin, lace-like structures, lying 

 in the sub-cutaneous connective tissue, and enlarge from defi- 

 nitely located " centers of ossification " by marginal additions. 



The scales of teleosts are developmentally and structurally 

 like those of ganoids, the ancestors of the group, but in form, 

 although often rhomboid in the young, they become approxi- 

 mately circular, and are hence termed cycloid. Ctenoid scales 

 are a variety of this in which the inner margin attached to the 

 skin is extended into numerous small processes like the teeth 

 of a comb. 



Modern amphibians have a soft, slimy skin, without exo- 

 skeletal structures of any kind, save in the rare order of ccecil- 



