VII.] 



THE EXTERNAL SKELETON. 



277 



form a peculiar function. The animal lives on eggs, but is 

 devoid of lips, so that if it broke the eggs with its mouth 

 their contents would be lost. It has then but rudimentary 

 teeth in the ordinary place, but swallowing the eggs whole it 

 fractures them with these cesophageal teeth, and thus loses 

 none of the nutritive substance. 



35. Structures which are quite like teeth and indeed much 

 resemble those of Orycteropus project from either side of 

 the very elongated rostrum which extends from the front of 

 the head of the Saw-fish. These, then, 



are teeth quite external in position 

 enderonic dermal calcifications. 



This, however, is not the only in- 

 stance in which tooth-like structures 

 appear on the external surface of the 

 body. The Sharks and Rays, for ex- 

 ample, have dermal calcifications 

 (springing from a bony base) which 

 consist of a substance exceedingly 

 resembling dentine, and moreover 

 coated with a sort of enamel. These 

 may be quite small and thickly dis- 

 tributed all over the body. A skin so 

 furnished is called shagreen. They 

 may however be larger, fewer, and 

 placed far apart, and as it were en- 

 graved with elegant patterns on the 

 exposed surface, often forming power- 

 fully defensive spines, as in some 

 Sharks. 



Other Fishes, as the Bony Pikes 

 {Lepidosteus)^ have a close - fitting 

 armour of solid, enamelled, bony scutes, 

 which join together by means of a peg 

 and socket articulation. 



Fishes more familiar to us, but belonging to the same 

 Ganoid group, e.g. the Sturgeon, are also furnished with bony 

 scutes arranged in rows along the body. 



The spines of some Teleostei present us with a peculiar kind 

 of articulation a shackle-joint (referred to in Lesson II.), 

 the base of a spine forming a ring which passes through 

 another ring developed from an ossicle supporting it (fig. 257). 



36. The SCALES OF ORDINARY FISHES (as the Perch) are 

 hard structures embedded in the deep layer, or dermis, and 



FIG. 256. UNDER SUR- 

 FACKOF HEAD OF A SAW- 

 FISH (Pristis), showing 

 the large lateral teeth on 

 the prolonged rostrum. 



