60 DB. W. G. RIDEWOOD ON THE CRANIAL [May 3, 



constitutes an ossification in the upper part of the interorbital 

 septum ; but in the Mormyridse and in Alhula 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 A'mia. As a rule the bone 

 is single, being formed by the confluence (although possibly not 

 during ontogeny) of idght 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 separ- 

 able ; 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 tei-m in the sense in which Owen 

 employed it, *. e. as a designation for the bone which is now more 

 commonly termed the alisphenoid (see Brit. Mus. Oat. Fishes, 

 ed. 2, i. 1895, p. 113, fig. 0, Percichthys:, and 'Poissonsdu Bassin 

 du Oongo,' 1901, p. Ii, Lates). 



The alisphenoid bones are usually separated, but in Notopteribs 

 they meet one another behind the orbitosphenoid. In Megalops 

 the alisphenoids unite above the brain. 



A basisphenoid is very generally present in the lower 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, Glianos, Mormyrus, and Mormyrops. 

 It is large in Albula, and assists the orbitosphenoid in forming 

 a complete bony interorbital septum (text-fig. 15, 0, p. 48). 



The eye-muscle canal (rnyodome of Ameiican writers) opens 

 posteriorly by a relatively large aperture, bounded right and left 

 by the posterior laminte of the parasphenoid, in Clupea, Dussu- 

 miei-ia, Ghirocentrus, and Engraidis, and in a somewhat similar 

 manner in Chatoessus, although the free wings of the parasphenoid 

 are here wanting. The canal also opens posteriorly in Uyodon, 

 Alhula, Elops, and Megalops ; but in the Osteoglossidse, the 

 Mormyrid^, and in Notoptems 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 posteiiorly 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 Oope has laid some stress (Trans. Amer. 

 Phil. Soc. n. s. xiv. 1871, pp. 454 &, 455), namel}^ the double 

 or simple nature of the basis cranii. This refers, so far as I 

 understand his writings, to the separation of the parasplienoid 



