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Show V particular character. The persistence of the interfrontal suture in the different genera of the Mursenidse, for instance, is very inconstant. In Osteoglossuvi the mesethmoid is separated from the frontals by the meeting of the two nasal bones in the middle line. In Megcdops the endosteal mesethmoid is more firmly united with the vomer than with the ectosteal mesethmoid ; in Arapaima the endosteal mesethmoid alone is present, and this does not present itself on the roof of the cranium; while in Chanos and the Salmonidse an ectosteal mesethmoid is present, and the cartilage is unossified. It is the rule among the Malacopterygian fishes for the two exoccipital bones to meet above the foramen magnum, and for the basioccipital to be excluded from the floor of the brain-cavity by the union of the two exoccipitals below the brain. The two pro-otics also unite beneath the brain, and form, with or without the co-operation of the basisphenoid, the roof of the eye-muscle canal. The supraoccipital crest is evidently to be regarded as an osseous sheet developed in relation with the great trunk-muscles, and not a backward extension of the supraoccipital bone itself. Chanos is very interesting in this respect, in that it shows a condition intermediate between the usual vertically disposed sheet of bone and the separable brush-like tendon-bone or intermuscular bone that projects back from the supraoccipital proper in Chatoessus. In the latter genus similar and separable osseous brushes project back from the epiotic bones; and in Sphyrcena and Mugil, Starks (Proc. U.S. Nat,. Mus. xxii. 1900) has described and figured similar brushes, but not separable, continuous with the back of the epiotic bones. Osseous brushes on the back of the exoccipital bones are of much more common occurrence, and these are discussed on p. 65. The extent to which the supraoccipital, epiotic, and squamosal crests project backwards, and to which the hinder surface of the cranium is excavated for the better attachment of the trunk-muscles, may be taken more or less as a measure of the specialisation of the skull, since in the pre-Cretaceous bony fishes the back of the cranium is nearly flat. The excavation of the back of the cranium has doubtless originated independently in different groups ; and Allis has pointed out (Zool. Bull. ii. 2, 1898, p. 92) that this must certainly have been the case in Amia and Scomber, for in the former the trunk-muscles have extended forward' beneath the parietal bones, whereas in the latter they lie externally to the parietals. The degree of irregularity of the back of the skull and the dimensions of the posterior temporal fossfe appear to be in direct proportion to the muscularity of the front portion of the trunk of the fish. The orbitosphenoid is a bone which is very variable in its form and occurrence; it is wanting in Osteoglossum, Gonorhynchus, and Chanos. In the majority of the lower Malacopterygian fishes it 1904.] OSTEOLOGY OF THE ELOPID.E AND ALBULID^E. 59 |