78 



LOWER VERTEBRATES. 



' chum,' and then this is converted into fertilizers. Experiments have also been tried 

 to make the meat into food for poultry. In olden times, in the treeless districts of 

 Cape Cod, the fishermen occasionally dried the dog-fish, and used them as fuel. 



On acconnt of the large numbers which are caught, this species is one which attracts 

 the attention of the embryologist. During the months of July and August, almost 

 every female contains a couple of transparent horny capsules, each containing from 

 two to four eggs, and hence supplies for study are obtained with comparative ease. 

 The embryos, in all stages of development, will live for some little time after being 

 taken from the parent, if kept in salt water. The older embryos, when about six or 

 ten inches in length, are spotted with large, round, white markings, which become 

 much less conspicuous, or even disappear, in the adult. 



The family Peistiophoeid^, ^Yith its single genus Pristiophorus, and its four 

 species, occurs in the Australian and Japanese seas. Tliese forms ha^'c the front of 

 the head prolonged into a very long flat blade, along each side of which is a series of 

 teeth. This gives these sharks much the appearance of the common saw-fishes (which 

 belong among the rays) but the differences between the two can readily be seen in the 

 position of the gill-clefts. These saw-bearing sharks are smaller than the saw-fishes, 

 and have a pair of long tentacles inserted at the lower side of the saw. They have no 

 anal fin, and no nictitating membrane, but the spiracle is present. Sqitaloraia, a liasic 

 genus, is supposed to belong near this family. 



The Cesteacionid^ is especially interesting to the geologist, because its members 

 are so abundantly represented in the older rocks ; twenty-two of the twenty-five known 

 genera having lived in the ages preceding the oolite. The living genera are but 

 poorly defined. Among the prominent characters of the family may be mentioned 

 the existence of two dorsal fins, each with a spine, the presence of an anal fin, a small 

 spiracle, and no nictitating membrane. The teeth are of several kinds, those in front 

 being smaller and more acute than those farther back, which take on a pavement-like 



character. These molars are ar- 

 ranged in oblique rows, which 

 vary in number and character, 

 forming the basis of the division 

 into genera. The mouth and 

 nasal cavities are confluent. 



The common name for the 

 members of this family is Port 

 Jackson sharks. All are confined 

 to the Pacific Ocean, one species, 

 Oyropleurodus francisci, occur- 

 ring on the coast of California. The best-known genus is Ileterodontus (= Cestracion). 

 None of the four living species exceed five feet in length, but the extinct forms reached 

 a much larger size. The oldest genus is Ctenoptychius., which appears in the Devonian 

 rocks, and is succeeded in the carboniferous by a large series of forms. Judging by the 

 character of the teeth, the members of this family live largely upon molluscs and crus- 

 taceans, though the- more acute teeth in the front of the jaw would seem to indicate 

 that these forms were not the whole of their food. 



The RHiNODOJfTiD^ embraces only two species of lajrge sharks, one of which well 

 deserves the name whale-shark, which is applied to it. This species, Rhinodon typl- 

 cits, of the Indo-Pacific region, "is known to exceed a length of fifty feet, but is 



Fig. 61. — Ileterodontus galeatus, Port Jackson stark. 



