44 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



surface of which secretes an even harder layer of enameP upon the 

 dentine base, this being thickest on the tip of the spine. The 

 mesenchyme in the papilla is the so-called pulp. With continued 

 growth the spine projects through the epidermis, giving the skin 

 of the shark its characteristic rough (shagreen) condition. This is 

 the placoid type of scale. 



FISHES. — In the adult elasmobranchs the scales may be large and remote 

 from each other (skates) or small and closely set. In the torpedo scales are 

 lacking, while in the chimxroids they occur only on the claspers, on the frontal 

 horn, and as extreme forms, in a great spine in front of the dorsal fin. 



Fig. 35. — Ventral armor of Stegocephals (after Credner-Zittel). A, Branchiosaurus; 

 B, detail of same; C, detail of Archegosauriis; Z), of Petrobates. 



A few ganoids lack scales (Polyodon), while the sturgeon have minute gran- 

 ules and five rows of large plates along the sides. Atnia has scales of the cycloid 

 type, soon to be described. With these exceptions the ganoids have ganoid 

 scales, which are rhomboid in outline and joined to each other like parquetry. 

 They consist of two layers, the lower apparently homologous with the dentine 

 of sharks, except that it is formed in, not on, the corium. The outer layer of 

 ganoin is formed by the corium and consequently cannot be enamel as once was 

 thought. 



A few teleosts are scaleless (some eels), but elsewhere scales are formed in 

 pockets in the corium (fig. 203). At first they lie side by side, but with growth 

 they overlap like shingles. There is only one layer of bone mixed with a large 

 amount of ossein (p. 27). In cycloid scales the element is circular and is marked 

 with concentric and radiating lines, the former indicating periods of growth. 

 The ctenoid scales differ in having the posterior edge of each scale truncate and 

 this edge and the surface toothed. Cycloid and ctenoid scales intergrade and 

 both kinds may occur on the same fish (many gobiids). 



^ There is some question whether this layer is really enamel; the usual statement as 

 to its nature is followed here. 



