A treatise on zoology . rostrum is drawn out into a longflattened blade bearing a single series of large teeth along its lateral edge(Fig. 116). Each of these modified denticles has a persistently growingbase deeply embedded in a socket in the hard calcified cartilage of therostrum. The Cretaceous genus Scleroi-hynchus (A. S. Woodward [503])shows a more primitive condition, in which the rostral teeth are smalland lie in the skin. Jaekel [238] considers that the shark-likeshape of the body has been secondarily acquired, and that the Pristidaeare derived from the Rhiuobatidae. In no other way ca
A treatise on zoology . rostrum is drawn out into a longflattened blade bearing a single series of large teeth along its lateral edge(Fig. 116). Each of these modified denticles has a persistently growingbase deeply embedded in a socket in the hard calcified cartilage of therostrum. The Cretaceous genus Scleroi-hynchus (A. S. Woodward [503])shows a more primitive condition, in which the rostral teeth are smalland lie in the skin. Jaekel [238] considers that the shark-likeshape of the body has been secondarily acquired, and that the Pristidaeare derived from the Rhiuobatidae. In no other way can we accountfor the condition of the pectoral fins, branchial slits, and vertebralcolumn, characters acquired apparently in adaptation to a mode of lifewhich the Pristidae have abandoned. , A. S. \V.; Cretaceous, Syria. Propristis, Davis;Eocene, Egypt. Pristis, Lath. ; tropical and subtropical seas ; Eocene,Europe and N. America. Family Rajidae. The adaptation to bottom living is carried ta RAJIFORMES i6i. almost its extreme condition (Fig. 129, A). Tiie slender tail is sharply marked off from the large disk. Small dorsal fins are usually carried near its extremity, and the caudal is much reduced or absent. The tail is triangular in section, with paired longitudinal keels; -^.nc some of its muscles are usually converted into an electric organ (Fig. 124) (Ewart[132]). Therhombic disk is formed chietly by the huge pectoral fins, whose anterior axis and radials reach forward to the pre- orbital process and beyond. In Platyrhina they almost, and in Symptenjgia they quite meet in the middle line. The basihyal is a very slender transverse rod, and the basibranchials are fused to a simple broad plate, i:)erhaps including some hypobranchials. The pelvic girdle bears a pair of prepubic processes. In Cydohatianot only these but also the iliac processes are very long. The pelvicfins are deeply notched, an anterior lobe being developed which issupported by specially enlarged
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
