OCR Text |
Show 1904.] OSTEOLOGY OF THE ELOPIIXE AND ALBULID.-E, 39 dorsal view of the cranium. The lateral temporal groove lies over the postfrontal bone, and is partially roofed over by a laminar extension of the squamosal. The subtemporal fossa, beneath the facet for the reception of the posterior head of the hyomandibular, is deep and extends inwards and upwards beneath the floor of the posterior temporal fossa. Its roof is formed by the squamosal, its floor and sides by the pro-otic and exoccipital, while its internal caecal end is formed by the supraoccipital. The opisthotic is small, and sends a process forward, below the subtemporal fossa, to meet a backwardly directed process of the pro-otic. The basisphenoid is of fair size. Its body is embraced by the alisphenoids and pro-otics, and it has a vertical descending plate which divides the eye-muscle canal and touches the parasphenoid by its lower edge. An orbitosphenoid of moderate size is present, but the greater part of the interorbital septum is membranous. The parasphenoid, which bears a spearhead-shaped patch of line teeth, extends back as far as the occipital articulation, but does not project beyond. The eye-muscle canal opens posteriorly by an aperture of moderate size. The vomer has fine teeth, disposed in two patches, right and left. The ethmoid region is very largely cartilaginous. The prefrontals are purely ectosteal, while the mesethmoid is clearly of double origin, the upper part (supra-ethmoid of some authors) being a membrane-bone, while the lower part, of diminutive size, is a cartilage-bone, separating without much difficulty from the former, but firmly united with the vomer. Temporal Series.-The supratemporal is a large thin lamina of bone with a sensory canal running along its anterior edge, which edge is in contact with the posterior edge of the parietal and squamosal bones. The hinder border of the supratemporal is deeply notched, which gives at first sight the impression that the bone is double. The mesial edge of the supratemporal lies over the supraoccipital crest, while the lateral edge overlaps the upper part of the opercular bone. The meeting of the two supra-temporals in the middle line of the head is noteworthy, and is reminiscent of Amia. The post-temporal has one limb loosely bound by fibrous tissue to the top of the epiotic, and a shorter and more slender limb attached to the back of the opisthotic. The third limb, lying beneath the supratemporal, is very short, and just fails to reach the outer edge of the posterior temporal fossa; but projecting forward from it is an ossified tendon, which terminates in a kind of brush in the middle of the fossa, as in Albula. Circmiorbital Series (text-fig. 9, p. 40).-The nasal aperture is surrounded by three bones ; the orbital ring is complete, and consists of seven bones. Maxillary Series (text-fig. 9, p. 40).-The gape is large, bounded above by the premaxilla and maxilla, both of which bear densely-set minute teeth extending along the whole of their lower borders. There are two surmaxillary bones. |