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Bashford Dean Memorial Volume 



Text'figure 7- 



Three different views of a tooth of Chlamydoselachus, six times natural size: 



A, seen from above; B, from the side; C, from beneath. 



After Garman, 1885.2, Figs. 1, 3 and 4, pi. VI. 



The essential similarity of the internal structure in scales and teeth of sharks is 

 evident from a comparison of Text-figure 8 with Text-figure 9. Each has the form of 

 a hollow cone, slightly recurved at the apex. Each is composed of dentine (D., D.2) 

 overlaid with enamel (e., S.). The dentine is traversed by canals (d. c.) radiating from the 

 pulp cavity (p. c. and P.). 



Both scales and teeth are exoskeletal structures. Evidently teeth, which are the 

 more complex, have developed from the same materials and in the same manner as scales. 

 It would, perhaps, be a trifle crude to say that teeth are developed from scales, but it 

 seems entirely proper to say that teeth are homologous with scales. This has long been 

 admitted, but in Chlamydoselachus we have material exceptionally favorable for revealing 

 the precise manner in which teeth correspond to scales. Superficially, the chief difference 



Text-figure 8. Text-figure 9. 



Sagittal sections showing similarity of structure between scales and teeth of sharks. 



Text-figure 8. Section showing finer structure of a placoid scale of Sc)'m?ius lichia. 



c.c. central canal; d.c, dentinal canal; e., enamel; p.c, pulp cavity. 



After Daniel, 1934, Fig. 35; redrawn from Hertwig, 1874, Fig. 2, Taf. XII. 



Text-figure 9. Section of a single-cusped tooth (x 75) from the lower jaw of a 

 340-mm. embryo of Chlamydoselachus. 



D., dentine; D.2, strongly calcified dentine; P., pulp cavity; S., enamel; So., base. 

 After Rose, 1895, Abb. 9. 



