EXISTING ORGANISMS— ANATOMY 



69 



to it is added the series of dermal bones which encloses the brain 

 on all other sides and form the greater part of the skull (Fig. 37). 



The Ganoid Stage. The stage in which the dermal bones of 

 the skull are well developed is nicely represented by existing bony 

 fishes, or ganoids, and hence is called by Wilder the ganoid stage. 

 The sturgeons illustrate the origin of these bones as dermal plates, 

 or scutes, which are derived like scales 

 from the corium, the under layer of 

 the skin, and instead of forming an 

 inner bony box, remain an outer bony 

 armor (Fig. 45). Wilder describes 

 these scutes as follows: "The snout, 

 or rostrum, is covered by a series of 

 small rostrol plates, which extend back 

 as far as the nostrils; back of these 

 openings may be found a pair of nasals; 

 behind these again, and between the 

 eyes, is a pair of frontals, often ac- 

 companied by prae- and post-frotitals. 

 Behind these is a pair of parietals, 

 and one or more supra-occipiials. On 

 the sides of the head, at about the 

 level of the parietals, are the squamo- 

 sals, and around the eye are several 

 orbitals, distinguished as pre-, supra-, 

 post-orb itals, etc. The operculum, or 

 gill-flap, which is present in these 

 fishes, is covered and augmented by 

 supra-, sub-, and pre-operculars." 



The occurrence of bones similar in 

 arrangement to these bony scutes is 

 dependent to some extent, of course, 

 on their functional importance in the 

 various fishes. Terrestrial animals, for 

 example, would not be expected to 

 have bones corresponding to the opercular series, and the shorten- 

 ing of the face and great enlargement of the brain in man must 

 necessarily be accompanied by differences in the skeletal parts 

 involved. Due to these variations in importance, exact duplica- 

 tion in widely different species is not found. 



Fig. 45. — Dorsal view of the 

 skull of a sturgeon {Acipen- 

 ser), showing dermal bones. 

 ROS, rostral plates; N, nasal; 

 F, frontal; PrF, pre-frontal; 

 Post Fr, post-frontal; Post 

 Orb., post-orbital; P, par- 

 ietal; SQ, squamosal; OP, 

 opercular; OCLa, lateral oc- 

 cipital; SO, supra-occipital; 

 SCL, supra-clavicle. (From 

 Wilder's Hislory of the Hu- 

 man Body, with the permis- 

 sion of Mrs. H. H. Wilder 

 and Henry Holt and Com- 

 pany.) 



