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Show 58 DR. W. G. RIDEWOOD ON THE CRANIAL [May 3, the parietal bones during evolution must be borne in mind ; and if the Berycoid fishes are to be regarded as the parental stock of the Acanthopterygians generally, a view which is supported by both comparative anatomy and palaeontology, all instances of contact between the parietal bones among the Acanthopterygian fishes (e. g. Cyttidse, Scorpaenidae, Triglidas) are to be looked upon as secondary. In support of the plausibility of the hypothesis may be mentioned the parallel instance of union of epiotic bones in the middle line in such forms as Lophius, Regcilecus, and Luvarus, this union being without doubt of secondary origin. In Chcinos there exists a condition which is calculated to make one pause before concluding that even within the limits of the Malacopterygian fishes the meeting of the parietals necessarily indicates the retention of the primitive condition ; for in this genus the parietals are widely separated in the young, but by subsequently fusing with the scales of the commissural section of the sensory-canal system, they come to meet above the supraoccipital bone*. They do not meet in an extensive suture ; indeed, they leave a considerable portion of the supraoccipital exposed both in front and behind ; but the condition is just sufficient to make it advisable to trace the development of the roofing-bones of the cranium in those forms in which the meeting parietal bones are thin, and beneath which bones the supraoccipital extends a considerable distance forward. This last relation, it may be observed, is the rule rather than the exception. Boulenger has already alluded to it in the case of the Salmonidae (Proc. Zool. Soc. 1895, p. 300), and I gather that he regards the union of the two parietal bones as secondary if the supraoccipital bone can be shown to extend beneath them so as to touch the frontal bones. But if this be so, the condition found in those primitive genera Elops and Albula must be secondary, for in them the supraoccipital touches the frontals beneath the united parietal bones; a fact evidently overlooked by Boulenger when he drew up his synopsis of the families of Teleostean fishes (Ann. & Mag. Nat. Hist. 1904, vol. xiii. p. 164). In Clupea, Chatoessus, and Chirocentrus the cranial roof ie> deficient towards the front of its upper surface, and a fontanelle occurs between the frontal bones and the mesethmoid. Perhaps this tendency for the frontals to remain apart points to some affinity with the Characinidae and Cyprinidae. In Citharinus such a fenestra extends the whole length of the frontal bones and involves also the parietals, while in Alestes a fenestra is found between the parietals and the hinder part of the frontals. A condition similar to the last occurs in such Cyprinoids as Cato-stomus, Cyprinus, and Cobitis (see Sagemehl, Morph. Jahrb. x. and xvii.). The interfrontal suture is obliterated in Gonorhynchus, but it does not appear that much weight need be attached to this * Cope (Trans. Amer. Phil. Soc. n. s. xiv. 1871, p. 455) mentions " parietals united " as one of the primary distinctive features of the family Lutodiridae. |