172 



FISHES. 



adult fish except some ridges or radiating lines. These pro- 

 cesses seem to serve as weapons of defence during a period 

 in the life of the fish in which it needs them most. In 

 not a few instances a portion of this armature is so much 



Fig. 85. — Tliolichthys osseus. Six times the natural size. 



developed that the disappearance of its most projecting parts 

 with the growth of the fish is not only due to its being sur- 

 rounded by other bone, but, partially 

 at least, caused by absorption. The 

 Carangidce, Cyttidm, Squamipinnes, 

 Xiphiidce, offer instances of such 

 remarkable changes. A fish, de- 

 scribed as Tholichthys osseus (Fig. 

 85), is probably the young of a 

 Cyttoid, the supra-scapula, humerus. 

 Fig. 86.— Thoiichthys-stage of and prseopcrculum forming enor- 

 enioc us (.). mously enlarged plates. In the fish 



Fig. 86 those bones appear still enlarged, and the frontals 

 develop a remarkably long and curved horn above the orbit. 

 In the Tholichthys-stage of Pomacanthus (specimens 10 milli- 

 metres long, Fig. 87), the frontal bone is prolonged into a 

 straight lancet-shaped process, nearly half as long as the 

 body; the suprascapular and prseopercular processes cover 



