. Water reptiles of the past and present . •43 Q,, 9r S ? 91 ID P ~ OS ^ •O XI 3« u +Jnj cjj h 5^-5 3 n 28 WATER REPTILES OF THE PAST AND PRESENT is not needed. The arrangement of the dentine in primitive reptilesis complicated, that is, plicated or folded in labyrinthine figures,like that of many stegocephalian amphibians, the Labyrinthodontia,especially. This labyrinthine structure of the dentine persistedlongest in the ichthyosaurs. VERTEBRAE AND RIBS The spinal column or backbone of reptiles, as in all air-breathingvertebrates, is made up of a variable number of separate segmentscalled ver
. Water reptiles of the past and present . •43 Q,, 9r S ? 91 ID P ~ OS ^ •O XI 3« u +Jnj cjj h 5^-5 3 n 28 WATER REPTILES OF THE PAST AND PRESENT is not needed. The arrangement of the dentine in primitive reptilesis complicated, that is, plicated or folded in labyrinthine figures,like that of many stegocephalian amphibians, the Labyrinthodontia,especially. This labyrinthine structure of the dentine persistedlongest in the ichthyosaurs. VERTEBRAE AND RIBS The spinal column or backbone of reptiles, as in all air-breathingvertebrates, is made up of a variable number of separate segmentscalled vertebrae, permitting flexibility. Each vertebra is com-posed of a body, or centrum, and an arch on the dorsal side forthe protection of the spinal cord. Various projections from thevertebra, called processes, serve for the attachment of ligaments. Fig. 12.—Procoelous vertebra of snake: sa, zygantrum; zs, zygosphene; pz, pos-terior zygapophysis. or muscles, for articular union with adjacent vertebrae, or for thesupport of ribs, and these processes have characteristic differencesin different reptiles. The pair in front and behind, for articulationwith the adjoining vertebrae, may become obsolete or even lost inswimming reptiles, as we shall see; they are called not a few reptiles there is an additional pair for zygapophysialarticulation in front and behind, called zygosphene and zygantrum,for the greater strengthening of the column; they are especiallycharacteristic of snakes and certain lizards. In certain otherreptiles, especially the long-necked dinosaurs, there is an addi-tional pair arranged differently from the zygophene, that havereceived the names hyposphene and hypantrum. On the top of the arch is the spine or spinous process, whichmay vary enormously in size and length; sometimes it is flattened THE SK
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