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Show 1889.] EOCENE SILUROID FISHES. 203 other, so that the centra appear as one bone closely united to the basioccipital, and at their point of union with the latter bone there is a strong downwardly directed process. The Arius skull most closely resembling this fossil is one in the British Museum, which Dr. Gunther assures me is almost certainly Arius cjagorides. The correspondence between this and the fossil is so close as to leave no doubt as to their generic identity, and the following comparisons refer to this specimen. I have to acknowledge my indebtedness to Dr. Gunther, who on this, as on many other occasions, has spared no trouble in order to facilitate m y examination of the specimens under his charge, thereby enabling me to settle the affinities of this fossil fish in a manner which would not otherwise have been possible. The frontals of A. gagorides (fig. 8,fr.) differ from those of the Barton fossil in having the median cleft extended further backwards, and their hinder ends are proportionately "wider. The latter character is in relation to the form of the sphenotics, which are relatively narrower at their hinder part than is the case in the fossil. In ordinary bony fishes the post-temporal is connected with the skull by two processes, one of which is attached to the pterotic (squamosal) and the other to the epiotic; but the post-temporal of A. gagorides has in addition to these a third attachment by means of a long bar extending from its under surface to the base of the skull (fig. 9), and in addition to this there is a thin plate of bone, which may be a process of the post-temporal, extending under the epiotic to the transverse process of the vertebrae. The two upper processes of the post-temporal are ornamented on their upper surface with tubercles, and between them and the pterotic an opening is left which seems to vary in extent in different specimens. The bone to which the hinder of these processes is attached appears from its connections and relations to the auditory region to be the epiotic; but it may be the parietal. If the latter be the correct interpretation, then the ossicle behind it may be the epiotic; and the hinder process of the post-temporal certainly reaches thus far. On the other hand, if the parietal be absent and this bone be the epiotic, then the hinder plate will probably be a supernumerary temporal plate, wedged in between the wide hinder moiety of the supraoccipital and the epiotic. The post-temporal bone of Arius gagorides has the same structure and relations as that described in the Barton skull ; but the opening left between its two upper processes is not seen in the fossil. This feature, however, varies even in the recent species, and in the fossil the two processes evidently have joined and obliterated the space. The pterotic and parietal elements are larger than in the fossil, the last-named bone reaching to the margin of the bony cephalic shield, while in the fossil it seems probable that the epiotic and post-temporal bones excluded it from the margin. The supraoccipital of A. gagorides is an elongated bone constricted in the middle, the portion in front of the constriction corresponding with the part preserved in the fossil. In the main |
