OCR Text |
Show 742 m r . c. t a t e r e g a n o n [June 19, the three rods may be present, but short and not convergent anteriorly, or they may be entirely absent (text-fig. 118, 0 ). Text-fig. 118. Crania of Scymnorliinus (A), Carclicirias (B), and Orectolobus (C), seen from above. (A and B after Gegenbaur, C after Haswell.) r, rostrum; n, nasal capsules. The calcification of the vertebral centra, although subject to considerable modifications within the group, is nevertheless of great importance in determining the relationships of the families. The primary calcifications take the form of a series of double cones which constrict the notochord vertebrally. In the Odontaspididae, Lamnidae, and most Orectolobidae, the secondary calcifications, usually deposited as concentric laminae, radiate from these to the periphery in such a way as to leave four principal uncalcified areas running from the central double cone to the bases of the neural and haemal arches (text-fig. 116, A, B & C, p. 736). In Chiloscyllium and in the Scyliorhinidae and Carcliariidse, modifications of this arrangement result from a tendency for these calcifications to start, not from the central double cone, but from points nearer the periphery. In the Carchariidae and Scyliorhinidae, there may be developed four calcified rays, running from the central double cone into the four principal uncalcified areas above mentioned (text-fig. 116, C). The extreme of specialisation is reached in the Scyliorhinid genera Pristiurus and Pseudotriacis, in which the secondary calcification is represented only by a thin peripheral layer. The dorsal and anal fins have their cartilaginous supports typically well-developed, numerous and regularly arranged, with little tendency to fusion (text-fig. 120, A, p. 747). The pectoral fin in the Odontaspididae, Lamnidae, Scyliorhinidfe, and Carchariidae has perhaps a more primitive structure than in |