THE INTEGUMENT 677 



sutures, while the structures lying between the sutures are commonly 

 called the dermal bones of the skull. 



Frontals, parietals, maxillaries and squamosals . are found in all 

 higher groups, though the opercular and rostral series disappear entirely. 

 Most of the orbital series also disappear, with the exception of the lacri- 

 mal. Then, too, there are the dermal bones of the mouth cavity such 

 as vorners, palatines, and parabasal, which are supposed to have retained 

 the original character inasmuch as teeth often form on and in these 

 bones. 



In the higher forms the dermal bones, however, arise from various 

 centers of ossification in the cutaneous mesenchyme, and while this dif- 

 ference has been explained as a curtailing of the previous race history, 

 it is quite likely that there is little difference between dermal and car- 

 tilaginous bone formation in the highest mammalian forms, the dermal 

 being merely more "stretched out" portions, as will be learned when the 

 endoskeleton is studied. 



Scales in the teleosts, although often rhomboid when quite young, 

 become circular later and are then called cycloid. Ctenoid scales are 

 quite similar to cycloid except that they are set in diagonal rows in pock- 

 ets of the dermis with their free edges overlapping. (Fig. 401.) 



Amphibians do not have scales and hard exoskeletons, although there 

 are extinct forms in which the body was covered with them. 



In reptiles the scales arise only from the epidermis and .are there- 

 fore composed of horn or keratin. There is no trace of bone in them. 

 The corium, however, furnishes the nourishment to these keratin scales, 

 although it does not furnish any of the hard parts. There is no definite 

 knowledge as to the relationship between the scales of reptiles and those 

 of fishes. 



Reptiles also have other integumental structures beside the keratin 

 scales, namely : spines, combs, and claws ; all, however, also made of 

 keratin. 



The birds are structurally and developmentally quite like the rep- 

 tiles in that they possess feathers which are homologous to the reptile 

 scales, and in having their beaks and claws composed of keratin. There 

 have been toothed-birds in the past, and it is said that tooth-germs have 

 even been found in the embryonic jaws of some of our modern birds. 



In mammals, while there are tiny scales covering the body, these 

 are seen as definite hard structures mainly on the claws, tails, and some- 

 times on the backs of such animals as the armadillos (Fig. 387). They 

 are always only epidermal in origin. In the armadillos the corium sec- 

 ondarily supplies the hardening portions so that the covering of those 

 animals becomes very thick, hard, and osseous. 



It is assumed, very often, that formerly all mammals were covered 

 by hard scales because the hair arrangement of mammals is quite like 

 that of the scales. For example, on the tail of a rat, the scattered hairs 



