The remaining asterospondylous sharks form a sub-order, Galei, without dorsal spines, and with the vertebra more perfectly calcified. The principal family is the Gakorhinidce. Others are the Sphyrnidee or hammer-heads, Scyllior hiniche or cat-sharks, Ginglymostomidee, Hemi Orectolobide, Lamnide or man-eater sharks, Odontaspidie, or sand-sharks, Alopiida-, or thresher-sharks, Mitsukurinicke or spoon-bill sharks, Cetorhinidce or basking- harks, Pseu dotriakidor and Rhineodontida. t i thy Lam nide" and related families fossi teeth (Car charodon, Isurus, Lamna) are Tv numerous from the Jurassic to modern times The Tectospondyli have the vertebra each provided with secondary plates of calcified tis In the sub-class of HolocePhali or Chime roids the upper jaw or pterygoquadrate arcade is immovably joined to the skull. The teeth are coalesced into broad plates, and a fold of skin covers the gill-clefts so that there is but one external opening. The vertebral axis is imper fectly segmented, and the notochord is sur rounded by partially calcified rings. In all recent genera, and in most others, there is a strong spine in the first dorsal, and in the male the forehead has a singular cartilaginous hook with a brush of spines at the end.
There are fragments referred to the skeleton of a cliimaroid found in the Lower Silurian at Canon City, Col. Numerous forms appear in the Devonian. Four genera, representing three families, are now extant, the Rhinochsme ridce (Rhinochimara and Hariotta) in the deep seas, Chimeride or elephant-fishes (Chimera) in thc north and south temperate seas, and Callurhynchidce (Callorhynchus) in the seas of the southern hemisphere ly. Extinct families are the Ptychodontidw mixt), the Squaloraii de (extinct), and the /If', xanthide (extinct). Numerous extinct genera, are referred to the Chinn ride. Fossil fin-spines of many species of sharks and chimaroids, fishes otherwise un known, occur in the rocks. These are called sue concentrically arranged in one or more series. In these sharks there is no anal fin. To these belong the Squalide or dog-fishes, Dalatii da, and Echinorhinicke or bramble sharks, these families forming together the sub order Cyclospondyli, having the vertebral cen trum a simple constricted cylinder pierced by the notochord. To the Tectospondyli•proper we may refer a few families of sharks, the Squat inida (Rhinida) or monk-fishes, and Pristio phoride or saw-sharks. A third sub-order, Batoidei, includes all the skates or rays. These agree with the true Tectospondyli in having a number of series of concentric plates within the vertebra. The body is, however, more or less depressed, the broad pectoral fins outlining a body disk, and the gill-openings lie underneath instead of being lateral, as in all the sharks.
The rays are first certainly known from the Jurassic, although several of the Carboniferous shark have ray-like teeth, and have been re ferred to the group of rays.
The recognized families of rays are the Pristidide or saw-fishes, the Rhinobatide or guitar-fishes, the Narcobatide or torpedos, the !Wide or skates, the Dasyatidce or sting-rays, the Myliobatida or eagle-rays, the Mobulida or devil-fishes, and the Ptychodontida (extinct) of the Cretaceous. The earliest of these groups, the Rhisobatide date from the Jurassic.
ichthyodoroulites, and their proper classification is often a matter of much uncertainty. The earliest of these are known as Onchus, occur ring in the Upper Silurian.
Class kstracophori—The earliest verte brates actually recognized as fossils are known as ostracophores ( ' o'crrpcsicau , a box: 9soptu, to bear). These are most extraordinary creatures, which may be described as jawless, limbless, enveloped in a coat of mail. While they have been called mailed lampreys, the likeness to lampreys is almost wholly negative, resting in the total absence of jaws, limbs and limb-gir dles. What the mouth was like can only be guessed, but no trace of jaws has yet been found in connection with it. The most remarkable distinctive character is found in the presence of a hard shell, made of bony plates covering the anterior part of the body, while the backbone is developed as a persistent notochord, imper fectly segmented. The entire absence of jaw structures, as well as the character of the arma ture, at once separates them widely from the mailed arthrodires of a later period.
This group was originally called Ostraco dermi, a name preoccupied for the group of bony trunk-fishes (Ostraciide). The names Protocephali and Aspidoganoidei have also been used for them. The still earlier name Placodermi included the Arthrodires as well.
It may be that they are really allied to certain early crustacean-like forms which they resemble.
The ostracophores are found in the Ordovi cian, Silurian and Devonian rocks, after which they disappear. The species are very numer ous and varied. Their real affinities have been much disputed. Traquair regards them as much modified allies of ancient sharks, which view of the case is supported by features in the structure of the most shark-like of the orders, Anaspida. The absence of jaws and limbs separates them widely from true fishes, and there is no clear evidence in the structure of the fins and fin-supports that these structures Cephalarpidcr (extinct), Thyestida (extinct), and Odontodontidce (Tremataspidte) (extinct), with many genera and species.