68 The Philippine Journal of Science 1923 



by whose agency the semen is transmitted into the oviducts, 

 impregnation being therefore internal in all sharks except those 

 of the genus Somniosus, which includes two species of arctic and 

 subarctic seas. 



The eggs are few and large and may be developed externally 

 when, with the exception of those of Somniosus, they are covered 

 with a thick leathery or horny skin, or case, or they may be 

 developed within the body of the mother. The cases of some 

 shark eggs are spirally twisted, others are quadrangular with 

 each of the four corners produced into a long filament. The 

 egg cases of rays are wheelbarrow-shaped, with four "handles." 

 These handles and stringy filaments serve to attach the eggs 

 to fixed objects. In some species of rays the egg cases contain 

 several eggs, each of which may develop. 



In many sharks, and in some rays, a part of the oviduct 

 becomes enlarged into a uterus, and in some of the sharks a 

 placenta is formed similar in appearance to that of mammals 

 but different in development. 



The skin of sharks and rays may be naked, or it may be 

 provided with bony or horny plates, but usually it is covered 

 with placoid scales. These scales are minute, closely set spines 

 situated on a broader base, each spine consisting of dentine 

 covered with enamel; the base is composed of bone and the whole 

 scale therefore has the same essential structure as a tooth. 

 Placoid scales are usually so firmly attached to the skin that it 

 is very difficult to remove them, while they are so hard they will 

 take a stonelike polish. Formerly large quantities of shark 

 skins were used for polishing wood and ivory; their fineness, 

 hardness, and durability made them far superior to any other 

 abrasive available for work of the highest quality. Even in 

 spite of the great improvements in making emery and sand 

 paper, cabinet makers still use some shagreen. 



The principal use made of shark skins for many years has 

 been in the manufacture of sword grips and sheaths, card cases, 

 jewel boxes, and other small articles. Within the past five years 

 improved methods of removing the scales and of tanning shark 

 skins have been perfected and a rapidly increasing shark 

 leather business is being developed. Shark skins are nonporous 

 and therefore make a leather practically water and air proof. 

 The last-named quality is a serious defect in the manufacture 

 of shoes, but does not impair the value of the leather for many 

 other purposes. 



