40 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
salts on their outer ends, thus forming a small plate of dentine (p. 24) 
with a central spine into which the papilla extends. The overlying 
epidermal cells form an enamel organ, the lower surface of which 
secretes an even harder layer of enamel’ 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 chimzroids they occur only on the claspers, on the frontal horn, and 
as extreme forms, in a great spine in front of the dorsal fin. 
Fic. 32.—Ventral armor of Stegocephals (after Credner-Zittel). A, Branchiosaurus; B, 
detail of same; C, detail of Archegosaurus; D, of Petrobates. 
A few ganoids lack scales (Polyodon), while the sturgeon have minute granules 
and five rows of large plates along the sides. Ama 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. 181). 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. In cycloid scales the element is circular and is marked with concentric 
and radiating lines. The ctenoid scales differ in having the posterior edge of 
1 There is some question whether this layer is really enamel; the usual statement as to 
its nature is followed here. 
