Page:The New International Encyclopædia 1st ed. v. 18.djvu/73

* SHARK. 55 SHARK. are found in all seas, but are most abundant in the tropics. They are nearly all marine, a few entering fresh water, and one species living con- tinually in Lake Nicaragua. A TYPICAL SHARK'S EGG. The rough skin of sharks is employed by join- ers for polishing fine-grained wood, and for cov- ering the hilts of swords, tools, and the like, to niake them firmer in the grasp. (See Sha- green.) The fiesh is coarse, but is sometimes eaten. The fins abound in gelatin, and are much used by the CJiinese for making a rich gelatinous soup. The liver yields a large quantity of valu- able oil. .See Oil-Shakk. The sharks embrace several families, among which prominent ones are the Hexanchidse (cow- sharks), Ccstraciontidae (Port Jackson sharks), Heterodontidie (bull-head sharks), (iinglymosto- matidse (nurse-sharks), Galeida; (dog-sharks, topes, tiger-sharks, man-eaters, requiems, etc.), Sphyrinidae ( hammer-heads ), Alopiida; ( thresh- ers), Carchariid:F (sand-sharks), Lamnidae (or beagles), Cetorhinidie (the basking-sharks ), and Squalida; (dog-fishes). Most of these will be found described luider their common names. Consult the authorities mentioned under Fish. See Plates of Gbeat Sharks; Lampreys and DocuisH ; Philippine Fishes. Fossil Shark. Fossilized remains of sharks occur from the Lower Devonian upward, and even in the Upper Silurian detached fin-spines, teeth, and dermal denticles resembling those of elasmobrauchs are found, being thus among the earliest known remains of vertebrates. The re- lationships of these Silurian forms are doubtful, however, and some of them (the C(Elolepida in- cluding Lanarkia and Thelodus) possibly have closer affinities with the remarkable group of ostracoderms than with elasmoliranchs. From the Devonian upward undoubted sharks are met with, many known only' from fragments of the dermal structures — teetli, shagreen denticles, and fin-spines. These spines, when not definitely as- signable to an.y genera, are termed 'iclithyodoru- lites.' In a few cases the cartilaginous endo- skeleton is hardened by deposition of phosphate of lime-^calcified — so that jaws, vertebrae, fin- structure, etc.. are readily fossilized. Elasmo- branch paleontology, which may be said to have originated in the work of Louis Agassiz, has demonstrated that the sharks and rays of the present time represent but an insignificant rem- nant of a group which attained its maximum degree of differentiation and specialization as early as the Carboniferous. The characteristic forms of the Paleozoic, however, the primitive as well as the highly specialized, died out in the Permian, and their descendants of the Slesozoic have persisted to the present with little change. The most primitive nf fossil plasmobranchs are included in the order Pleuropterygii (side- fin) of which the most typical genus is Cladose- lache from the Upper Devonian or Lower Carbon- iferous of Ohio. In this form the paired fins are mere horizontal lappet-like folds along the sides of the body, supported by two rows of cartila- ginous rods, the 'basals,' imbedded within the body, and the 'radials' within the fin-lappet and extending outward to its edge. According to the commonly accepted fin-fold theorj' of paired lindjs, this is the most primitive known type of paired fin, and the lappets are to be regarded as persistent portions of a former continuous lateral fold, possessed by some unknown ancestor. Since these lappet-fins, or 'plenropterygia,' were capa- ble of but very slight motion, their function was chiefly that of balancing-organs, while the power- ful turned-up or 'heterocereal' tail served as the organ of propulsion. Other primitive characters of this fish are the terminally placed mouth, the unconstricted notochord, and simple dermal skeleton. Clodoselache, judging from its many primitive characters and lack of specialization, ])robably stands structurally verj' near the an- cestral form which gave rise to' the more spe- cialized sharks, to the bony fishes, and through these to the higher vertebrates. Several f'ladose- lachids are known, and the most generalized of these may be regarded as the most primitive true fish. None of them exceeds six feet in length. The spiny sharks (commonly ranked as an order Acanthodii ) comprise a 'number of Paleo- zoic forms which resemble the cladoselacliids in many respects, but differ from them in that the blade of the fins, except the caudal, is almost en- tirely dermal, the skeletal fin support being re- duced to a stiff spine at the anterior border; genera Acanthodes and ^Mesacanthus. In one family, represented by Climatius, a series of spines along the side of the body suggests the continuous lateral fin-fold. The acanthodians have the dermal skeleton highly developed, espe- cially in the region of the skull and shoulder girdle. Some ichthyologists place the group among the pleuropterygians. A widely different order of Paleozoic sharks is that termed lehthyo- tomi or Pleuraeanthea, represented Ijy Pleura- canthus of the Carboniferous and Permian of Europe. Of the many distinguishing features of this group, the most noteworthy is the possession of pectoral fins of the 'archipterygium' type, which many morphologists (Gegenbaur and" his school) maintain to be the fin-form from which are evolved all other types of paired fins, and even the five-toed limbs of higher vertebrates. In the perfect archipterygium the basals form an axis projecting from the body, while the radials are ranged along this axis iii two rows, like the veins of a leaf along the midrib. This type of fin is also common to the lung-fishes and some of the most primitive bony fishes. There is strong reason to believe that it is derived from the lap- pet-like type of the Pleuropterygii. (See Fin.) The elasmobrauchs thus far mentioned did not survive beyond the Paleozoic, but it is these early types only which are sufficiently primitive to be of importance in tracing the ancestry of higher vertebrates. The order Selachii, comprising all the modern sharks and rays, appeared in the Lias, though one family, the cestracionts, may be trace- able to the Permian. The basals of the pectoral fin are reduced to two or three pieces, and the blade of all the fins is chiefly dermal. The males are provided with claspers on the pelvic fins. The vertebral centra, with few exceptions.