EXOSKELETON 



II 



GANOIDS. — The scales of typical Ganoids consist of two layers. 

 The basal layer (usually called dentine, better, isopedin) resembles 

 true bone in having bone cells. The superficial layer (ganoin) is 

 denser and has a polished surface. Both layers are formed by the 

 corium and neither can be compared to the enamel of the placoid 

 scale. 



In the development of the scales of Lepidosteus the basal part is laid down in 

 the corium (fig. 8) ; it is at first circular in outline, and each scale remote from the 

 others. Then the basal layer of the epidermis deposits a true enamel as numer- 

 ous spines, these separated from the dentine by a layer of mesenchyme which 

 later secretes the ganoin, while spines and enamel are lost. It is a question 

 whether each ganoid scale equals one or several placoid scales. 



Fi(j 8.— .4. B, C, Development of scale of Lepidosteus (Nickerson, "93); D, adult 

 scales (Butschli, '10). A, early stage with development of organ for spine (compare 

 with development of placoid scale) ; B, later, with developing ganoin; C, margm of scale 

 of young fish showing two spines, d, dentine layer; e, epidermis; g, ganoin; m, mucus 

 cells; mc, mesenchyme (corium) overlying dentine; this develops the ganoin; o, odonto- 

 blast; p, pulp of spine. 



In the adult Lepidosteus the scales are enlarged to rhomboid 

 plates which shghtly overlap, forming a dense armor, the strength 

 of which is increased by an interlocking process on the anterior 

 margin of each scale (fig. 8, D). Polypterus (fig. 239) has scales super- 

 ficially much like those of Lepidosteus in appearance and arrange- 

 ment, but dift'ering in details (Goodrich, 07). The scales of 

 sturgeons (Chondrostei) are large plates of bone arranged, in living 

 species, in five longitudinal rows, the scales of the mid-dorsal row 

 being the largest. These arise as ossifications in the corium with- 

 out any enamel or ganoin, reaching the surface later by loss of the 

 overlying corium and epidermis. Besides these larger scales there 

 are smaller ones between the larger rows. Polyodon has scales in 



