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Show 70 DR. W. G. RIDEWOOD ON THE CRANIAL [May 3, nearest approach to such a condition is found in Notopterus, in which the two nasals are fairly large, and just meet in the middle plane of the head, and are with some difficulty removable from the underlying mesethmoid and prefrontal bones. A very similar condition obtains in Petrocephcilus, in which the nasal bones are exceptionally large. The postorbital cheek-plates are large in Arapaima, Osteoglossum, Hyodon, Elops, and Megalops. Both postorbital and suborbital bones are large in Albula. In Chirocentrus the bone lying antero-ventral to the orbit is large. There is one large suborbital, extending also behind the eye, in Chanos, Engraulis, and Clupea harengus, but in Clupea jinta the postorbital part of the cheek is protected by two large plates. In some forms, such as Coilia, Albula, and Notopterus, in which the postorbital and suborbital sensory canals are large, the bones have a scroll-like form ; but in the other genera the sensory canal lies farther from the orbital edge of the circumorbital bones (e. g., Clupea, Chatoessus), or is more deeply embedded in the bone (e. g., Arapaima, Chanos). In Notopterus there are processes of the two hindermost of the suborbital bones directed inwards below the eyeball towards the ectopterygoid and entopterygoid, with which they are united by ligament. There is also a process passing inwards from the anterior end of the front suborbital bone, and entering into close fibrous union with the under surface of the prefrontal. Boulenger finds that in some families of the Perciform fishes the " subocular shelf " is sufficiently constant to be of taxonomic value (Ann. & Mag. Nat. Hist. (7) xiii. 1904, p. 179). In this connection it is interesting to note that in Albula and in Dussumieria there is an outwardly-directed process of the ectopterygoid, underlying the eyeball, which meets the edge of the suborbital bones. This process, like the subocular shelf, serves the purpose either of supporting the eyeball, or of limiting the lateral play of the series of suborbital bones. In Osteoglosstom there is a close fibrous connection between the maxilla and the preorbital and suborbital bones, but this is not the case in Heterotis and Arapaima. Maxillary Series.-The evidence of palaeontology goes to show that the most primitive form of mouth is that bounded above by a small premaxilla and a comparatively long maxilla; at all events, an enlarged premaxilla has not yet been noted in Iso-spondylous fishes below the Cretaceous strata (Smith Woodward, Vert. Palaeontology, 1898, p. 113). Most of the forms now under examination have a premaxilla-maxillary gape ; but in Albida, in which, according to Jordan and Gilbert (Bull. U.S. Nat. Mus. No. 16, 1882, p. 258) and Smith Woodward (Brit. Mus. Cat. loss. Fish. iv. p. 59), the lateral margin of the upper jaw is formed by the maxilla, the maxilla can be of service only when the mouth is widely opened, and indeed, since it bears no teeth, while the premaxilla does, it probably does not function in |