Proceedings of the general meetings for scientific business of the Zoological Society of London, 1904 vol. 2

Page 67

44 DR. W. G. RIDEWOOD ON THE CRANIAL [May 3, nearly horizontally as it does in Flops. The parasphenoid does not extend as far back as the occipital articulation, but underlies the anterior two-thirds of the length of the basioccipital. The eye-muscle canal in two of the specimens examined opens posteriorly by an aperture which admits of the passage of a fairly large sewing-needle, but the canal is blind in the third specimen. The parasphenoid has a long narrow band of numerous small teeth, and the vomer bears a heart-shaped patch of similar teeth, com­pletely or incompletely divided into a light and left half. The endosteal mesethmoid is in rigid union with the vomer, and separates fairly readily from the ectosteal mesethmoid ; this gives the appearance of the vomer being in part a cartilage-bone. A lateral process of the ectosteal mesethmoid passes downward and outward to meet a forward process of the ventro-lateral bolder of the prefrontal below the nasal sac. This is not present in Flops. The parietal bones touch one another in the median line of the head; each is nearly square in shape. The supraoccipital does not spread far beneath the parietal bones, and does not come near the frontal.s. In the extinct species it appears to have extended farther forward (see Smith Woodward, Brit. Mus. Cat. Foss. Fish, iv. pi. iii. fig. 5; also page vi of the Introduction, in which the reference to Megalops was probably intended by the author to refer to the extinct species only). The posterior temporal fossa* are large and extend as far forward as the orbitosphenoid bone. They communicate with one another above the roof of the brain-case, although in Flops they are a considerable distance apart. The roof of the brain-case is formed by the alisphenoids (which meet in the middle line above the brain*) and by a forward growth of the lower part of the supra­occipital . The supraoccipital either actually touches the alisphenoids, or a narrow tract of cartilage intervenes. The roof of the poste­rior temporal fossa is formed by the parietal, squamosal, epiotic, and frontal bones. The postfrontal is rather hollow, and forms part of the external wall and floor of the fossa, and the pro-otic also forms part of the floor. The lateral temporal groove, above the articular facet for the head of the hyomandibular, is broad and shallow, and is not roofed over. The subtemporal fossa is deep, and extends inwards and upwards. It is bounded above by the squamosal, below by the exoccipital and pro-otic, behind by both squamosal and exoccipital bones, and in front by the pro-otic. The opisthotic is comparatively large. Its postero-superior part, to which the deep limb of the post­temporal is attached, is small and wedged in between the main part of the exoccipital and the part of this bone that forms the posterior border of the subtemporal fossa. The lower part of the opisthotic, however, extends forwards so as to form an important constituent of the side of the cranium. It is somewhat bullate in shape and touches the pro-otic and basioccipital. This bullate * Hay (Zool. Bull. ii. 1. 1898, p. 32) states that in Tarpon atlanticus the ali­sphenoids meet in the mid-line below the brain. This is not the case in the specimens now under consideration.