OCR Text |
Show GG DR. W. G. RIDEWOOD OX TIIE CRANIAL [May 3, the opisthotic limb of the post-temporal. In Osteoglossum also there are two important intermuscular bony brushes on each side. These osseous brushes are probably far more common in their occurrence than is generally suspected, and the reason that they have not attracted more notice in the past is probably due partly to the fact that anatomists have been disposed to discount their value as constituents of the skeleton (although they are just as important as epipleural bones, which are not disregarded when dealing with the vertebral column), and partly also because the preparateur dissects them away from the back of the cranium when removing the skull from the vertebral column, and they thus become thrown away with the muscles of the trunk. They have been noted, how'ever, in the Black Bass and the Tunny by Shufeldt, who calls them " occipital ribs" (Rep. U.S. Fish. Com. 1883 (1885), p. 805). Hyrtl (Denkschr. Akad. Wiss. Wien, xxi. 1863, p. 3), with considerable acumen, has likened those at the back of the skull of Chanos to the ossified tendons of birds. Such intermuscular brushes are not confined to the exoccipital and opisthotic bones. In Chatoessus they occur on the supraoccipital and epiotic prominences, and in Chanos the supraoccipital spine is produced back into a bony brush exactly similar to that of Chatoessus, except in that it is not a separate structure. In Gonorhynchus the posterior end of the outer intermuscular bone is connected with the post-temporal by means of fibrous tissue. There is no opisthotic limb of the post-temporal besides this, which is clearly the opisthotic limb which has failed to establish the usual osseous connection with the post-temporal. In Albula, in addition to a fully developed opisthotic limb, there is an ossified tendon which projects forward from the inner surface of the posttemporal bone into the posterior temporal fossa, where it branches among the fibres of the trapezius muscle. The supratemporal may in a general way be described as that dermal bone which receives the lateral line from the post-temporal and transmits it to the squamosal, and gives off a branch to the parietal (e. g. Clupea), or to one or more tubular scales of the transverse commissure of the sensory-canal system separable from the parietal (e. g. Albula. Salmo). The form of the supratemporal is thus triradiate. In Gonorhynchus, however, in which also the commissural scales are separable from the parietals and supraoccipital, the forking of the canal-system occurs just in front of the supratemporal, so that this bone is a plain tubular scale. The supratemporal is a bone which lies either above or posterior to the squamosal ridge, and either below or external to the epiotic ridge; and Collinge (Proc. Zool. Soc. 1895, p. 291) is undoubtedly right in questioning the correctness of Parker's application of the term supratemporal to that bone of the lateral-line system which in the Salmon lies above the preopercular and below the squamosal ridge (Phil. Trans, vol. 163. 1873, p. 99, and pi. 6. fig. 1, s<). Parker's " supratemporal " appears to correspond exactly with that bone which in Chanos lies immediately above the opercular |