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Show 1 9 0 6 . ] SELACHIAN FISHES. 73 5 especially in the case of genera of doubtful position, I have endeavoured to confirm or to extend the observations which have been made by others. The endoskeleton of the Selachii may be conveniently considered under the heads: (1) the axial skeleton, or cranium and vertebral column; (2) the visceral skeleton, comprising the labial cartilages, jaws, and branchial arches; and (3) the pterygial skeleton, comprising the supports of the median and paired fins, including the pectoral and pelvic arches and the mixopterygia. Modifications in structure of these three systems are considered in the account which follows. In all living Selachians the vertebral column is made up of the notochord and its cartilaginous sheath and of dorsal and ventral series of paired cartilages attached to the latter. These paired cartilages consist of principal pieces, neural plates (basi-dorsals) and haemal plates (basi-ventrals), and of intercalary pieces alternating with these, the interneural and interhaemal plates. Centra may be formed by the segmentation of the cartilaginous notochordal sheath. The neural plates are typically broad at the base and narrowed above and vertebral in position, whilst the interneurals are correspondingly narrowed below and intervertebral in position. Both neurals and interneurals may meet in the middle line and unite above the spinal cord ; or if the interneurals of one side are juxtaposed above the apices of the intervening neurals, then only the interneurals may so unite. A median series of cartilages may sometimes apparently be segmented off from the united paired elements. The neurals and interneurals may not meet above, and in that case the roof of the neural canal may be completed by a longitudinal strip of cartilage, apparently of independent origin, usually, but not always, segmented*. Sometimes the incomplete union of neurals and interneurals leaves a series of interspaces, which are filled by a median series of cartilages f. The plates composing the neural and haemal arches may each become secondarily segmented into two or more pieces +. The haemal plates correspond to the neurals and are vertebral in position, whilst the interhaemals are intervertebral. The latter are often reduced or absent, especially in the caudal region. In the praecaudal region the haemals remain separate and may bear rib-like cartilages, which are intermuscular in position and probably not homologous with the ribs of Teleostomi. In the caudal region the haemals unite below to form a complete haemal arch, and a median series of cartilages may be segmented off. Primitively the neural and haemal plates are loosely attached to the chordal sheath, but sometimes they become more intimately united with it, and * In a young Carcharias melanopterus I find an unsegmented median longitudinal rod of cartilage completing the neural canal. f The median cartilages of Scyliorhinus are probably not derived from the neural arches. X Compare Hasse's plates of various Hypotremata and also Helbing's figures of Lcemargus borealis and L. rostratus. P roc. Z ool. Soc.- 1906, No. XLIX. 49 |
