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Show 04 DR. W. G. RIDEWOOD ON THE CRANIAL [May 3, bones arise independently of the teeth, it is not unreasonable to assume that the presence of teeth on these bones indicates a more primitive condition than the absence of teeth-unless, of course, the teeth are of secondary, and not primary origin. A general review of the skull in Teleostean fishes certainly goes to show that, although no broad conclusions can he drawn from the presence or absence of teeth on the vomer, a dentigerous parasphenoid is habitually associated with other features which may be regarded as archaic. Of the genera examined, Elops, Megalops, Albula, Arapaima, Osteoglossum, Notopterus, Hyodon, and the Mormy-ridae* have teeth on the parasphenoid. There is probably in a considerable number of the lower Teleosteans the half of a vertebral centrum fused with the basi-occipital and exoccipitals to form the hollow-cone articular surface at the back of the cranium, so that the cranial articulation really takes place between this half-centrum and the first free centrum. Such a condition has long been known to exist in Amia, in which the corresponding neural arch persists; and Shufeldt (Rep. U.S. Com. Fish. 1883 (1885), p. 816) has shown that in Megalops the suture between the half-centrum and the exoccipitals and basi-occipital is readily recognisable. When this half-centrum is removed, the transverse end of the basicranial axis presents a rough surface, with a suture of the shape of an inverted T or Y separating the basioccipital below and the right and left exoccipitals above. In all of the forms at present under consideration, the upper part of the suture between the half-centrum and the exoccipitals is recognisable in the floor of the foramen magnum, although the suture may no longer be visible down the side of the half-centrum ; and in these cases the half-centrum can only be removed by the application of some force, and the inverted T suture above mentioned be brought into view. In the Salmonidfe, in Hyodon and in Heterotis, although there is not a very definite articulation between the first centrum and the basioccipital and exoccipitals, and only a very slight range of movement is possible between these, the centrum separates quite readily, and exposes the triradiate or inverted T suture, as already figured by Gegenbaur (Festschr. A. v. Kolliker, Leipzig, 1887, pi. i. fig. 9) and Parker (Phil. Trans, vol. 163. 1873 (1874), pi. viii. fig. 8). The condition thus approximates to that which is so commonly met with in the Acanthopterygian fishes, where the cranial articulation is definitely between the basioccipital and exoccipitals in front and the first vertebra behind. In Arapaima the occipital articulation is further complicated by the fact that the first complete centrum, although free from the half-centrum in front of it, is not capable of free play, since its lower portion sends forward a pair of long, stout processes which are rigidly united by oblique, jagged sutures with the back of the parasphenoid. * The parasphenoidal teeth are rudimentary in Mormyrops and absent in Gymnarchus. |
