| OCR Text |
Show 60 DR. W. G. RIDEWOOD ON THE CRANIAL [May 3, constitutes an ossification in the upper part of the interorbital septum; but in the M or my rid a? and in Albula and Arapaima it extends the full height from parasphenoid to frontal. In the latter genus it is a paired bone-a very unusual feature in Teleostean fishes, and reminiscent of Amia. As a rule the bone is single, being formed by the confluence (although possibly not during ontogeny) of right and left constituents, the union being greatest ventrally, so that the bone in transverse section has the appearance of a U, or a Y, or a T, or an I. Shufeldt, it is to be noted, speaks of orbitosphenoids in the plural in his description of the skull of Grammicolepis (Journ. Morph, ii. 2, 1889, p. 280), but it does not necessarily follow that in this statement he intends to convey the idea that the right and left parts are really separable ; Hay, for instance, speaks of orbitosphenoids ankylosed in the mid-line as in the salmon " (Zool. Bull. ii. 1, 1898, p. 32). It is to be borne in mind, further, in dealing with the orbitosphenoid that Boulenger still uses the term in the sense in which Owen employed it, i. e. as a designation for the bone which is now more commonly termed the alisplienoicl (see Brit. Mus. Cat. Fishes, ed. 2, i. 1895, p. 113, fig. C, Percichthys; and ‘ Poissonsdu Bassin du Congo,' 1901, p. Ii, Lates). The alisphenoid bones are usually separated, but in Notopterus they meet one another behind the orbitosphenoid. In Megalops the alisphenoids unite above the brain. A basisphenoid is very generally present in the lowrer Teleostean fishes, and has the form of a Y or a T when viewed from the front; but it appears to be wanting in Arapaima, Heterotis, Osteoglossum, Gonorhynchus, Chanos, Mormyrus, and Mormyrops. It is large in Albula, and assists the orbitosphenoid in forming a complete bony interorbital septum (text-fig. 15, C, p. 48). The eye-muscle canal (myodome of American writers) opens posteriorly by a relatively large aperture, bounded right and left by the posterior laminae of the parasphenoid, in Clupea, Dussu-mieria, Chirocentrus, and Engraulis, and in a somewhat similar manner in Chatoessus, although the free wings of the parasphenoid are here wanting. The canal also opens posteriorly in Hyodon, Albula, Elops, and Megalops ; but in the Osteoglossidse, the Mormyridse, and in Notopterus and Chanos it terminates blindly. The canal is open in the Salmonidse ; but owing to the large amount of cartilage present in the cranium, and the consequent shrinkage on drying, the appearances presented by the dried skulls are apt to be misleading. While some importance may perhaps be attached to the fact that the eye-muscle canal either opens posteriorly or terminates blindly-the facts stated in the last sentence are rather against this conclusion-no value can be ascribed, so far as I can see, to a feature upon which Cope has laid some stress (Trans. Amer. Phil. Soc. n. s. xiv. 1871, pp. 454 & 455), namely, the double or simple nature of the basis cranii. This refers, so far as I understand his writings, to the separation of the parasphenoid |
