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Show 62 DR. W. G. RIDEWOOD OX THE CRANIAL [May 3, homologous with the pre-epiotic fossa. If the pre-epiotic fossa of Clupea were to become larger and deeper, the exoccipital would probably form part of its margin. The relation of the supra-temporal bone to the lateral cranial foramen in the one case and to the pre-epiotic fossa in the other, suggests a morphological equivalence. The lateral temporal groove is fairly well roofed over in Albula, Chanos, and Heterotis, and paxtially so in Mops, Arapaima, Osteoylossum, Hyodon, and Notopterus. In Osteoylossum, Albula, Elops, and Megalops there is a depression of considerable size in the side of the cranium, situated beneath the posterior part of the articular surface for the head of the hyomandibular. This is the " subtemporal fossa " ; it is bounded by the pro-otic, squamosal, exoccipital, and opisthotic bones. The fossa is recognisable in Arapaima, but it is only a shallow depression. According to Hay (Zool. Bull. ii. 1, 1898, p. 30) it is well developed in the Cretaceous genus Xiphactinus, in which it was erroneously described by Cope as a foramen. As already shown by Sagemehl (Morph. Jahrb. xvii.) the subtemporal fossa attains to great dimensions in such Cyprinoid fishes as Cyprinus, Leuciscus, Labeo, and Abram/is-it is practically wanting, however, in Cobitis and Botia. In the former Cypri-noids it is situated more posteriorly with respect to the articular facet for the hyomandibular than is the case in the genera mentioned at the beginning of the preceding paragraph, and it opens vertically downward instead of downward and outward. It is so large and deep as to be separated from the dorsal surface of the skull by but a single layer of bone (epiotic bone). In these fishes the subtemporal fossae serve for the lodgment of the great muscles which, by pulling up the inferior pharyngeal bones (fifth ceratobranchials), bring the teeth upon those bones forcibly against the callous pad that is carried by the under surface of the basioccipital bone. The subtemporal fossa is not present in the Salmonidfe. It appears in the form of a wide but shallow depression in Pro-chilodus, Alestes, and Hydrocyon*, but the fossa is unrecognisable in the majority of the Characinidse. The " auditory fenestra," bounded by the pro-otic, exoccipital, and basioccipital, and leading into the perilymphatic cavity, is present in Chatoessus, Chirocentrus, Clupea, Dussumieria, Engraulis, and Hyodon. In the last-named genus it is traversed by a vertical bar of the pro-otic. There are csecal diverticula of the swim-bladder contained in the squamosal and pro-otic bones respectively in Chatoessus, Chirocentrus, Clupea+, Dussumieria, Engraulis, and Coilia. Megalops has a cavity in its opisthotic, which is probably occupied by a similar diverticulum, although I have not had an opportunity of tracing the connection with the swim-bladder ; but in Elops there * See, for instance, Sagemehl, Morph. Jahrb. x. 1, 1884, pi. 2. figs. 13 & 14. f In Clupea sprattns the pro-otic vesicle alone is present. |