OCR Text |
Show 80 DR. W. G. RIDEWOOD OX THE CRANIAL [May 3, present they may be fused to the bone, or may strip off with the mucous membrane, and leave no scar upon the surface of the bone. The matter is of some little importance because the mucous membrane is customarily allowed to dry on the pharyngeal skeleton to prevent the constituent parts from separating, and loose teeth thus appear as if they were rigidly attached. The dentigerous plates on the fifth ceratobranchial are readily removable in Elops and Megalops, which is rather remarkable, for the teeth of the fifth ceratobranchial are usually firmly fixed to the bone even in those cases in which there is considerable reduction in the hyobranchial dentition as a whole. It is worthy of remark in connection with the probable origin of the ectosteal constituent of the glossohyal by the coalescence of tooth-bases (cf. vomer, &c., p. 63), that in the Herring the glossohyal is a cartilage overlaid by the ectosteal lamina; and the teeth, although they may leave scars on the bone when the mucous membrane is stripped off, are not intimately attached to the bone. This evidently indicates a process of degeneration in the lingual dentition, the first stages in the transition to an edentulous state being marked by a reduction of the basal parts of the teeth. The dentigerous plates lying on the pharyngeal surface of the second, third, and fourth pharyngobranchials are not collected together to form an epipharyngeal apparatus as, for instance, in the Cod, but they remain distinct. The cartilage of the fourth pharyngobranchial remains unossified, but in some forms, such as Albula and Chirocentrus, the cartilage in drying shrinks upon the underlying dentigerous plate in such a way as to give the impression that itself is ossified. In Chatoessus the ectosteal bone, here toothless, spreads over the mesial and dorsal surfaces of the cartilage, and gives the effect of a completely ossified fourth pharyngobranchial. Swinnerton, it may be mentioned, has recorded the presence of a truly ossified fourth pharyngobranchial in Cromeria (Zool. Jahrb., Abth. f. Anat. xviii. 1903, p. 65, figs. H & K). Engraulis, Gonorhynchus, and Alepocephalus have a distinct fifth epibranchial cartilage. The presence of this element in Alepocephalus was pointed out by Gegenbaur, who has also figured one in Clupea alosa (Morph. Jahrb. iv. Suppl. p. 24, and pi. 2. fig. 13, Alosa vulgaris or Clupea vulgaris), lrr Clupea harengus and C. Jinta the position of the fifth epibranchial is occupied by a ligament; in most genera there is no representative of this element of the visceral skeleton. The epibranchial accessory organ of respiration produces, in those Malacopterygians in which it is at all largely developed, important modifications in the shape and size of the elements of the fourth and fifth branchial arches. In Chatoessus, and to a lesser- extent in Chanos, the fifth ceratobranchial and the fourth epibranchial are considerably increased in width, but in Gono-rhynchus these bones are only lengthened. For our knowledge of the structure of the epibranchial organ of Clupeoid fishes we are |