. Bulletin of the British Museum (Natural History) Zoology. ptss psk Fig. 7 Ompok bimaculatus: neurocranium in left lateral view (shown in correct orientation relative to vertebral axis); only those features of particular significance and discussed in the text are indicated (BMNH :171-8). mec. Fig. 8 Ethmoverine region in ventral view of, A, Ompok bimaculatus (BMNH :171-8); B, Kryptopterus bleekeri (LILI 89030). nerve (Figs 5, 12A). Ompok lacks this condition, as do other silurids investigated. Silurus, however, displays both condi- tions. In three specimens of S. glanis, o
. Bulletin of the British Museum (Natural History) Zoology. ptss psk Fig. 7 Ompok bimaculatus: neurocranium in left lateral view (shown in correct orientation relative to vertebral axis); only those features of particular significance and discussed in the text are indicated (BMNH :171-8). mec. Fig. 8 Ethmoverine region in ventral view of, A, Ompok bimaculatus (BMNH :171-8); B, Kryptopterus bleekeri (LILI 89030). nerve (Figs 5, 12A). Ompok lacks this condition, as do other silurids investigated. Silurus, however, displays both condi- tions. In three specimens of S. glanis, one has a single trigeminal opening, although it is partially divided by a bony spur extending anterodorsally from the prootic (Fig. 12B), and two have a secondary foramen for the hyomandibularis nerve formed by a complete suture between a prootic process and the pterosphenoid (Fig. 12C). In S. asotus a prootic spur extends ventrally from the dorsal rim of the trigeminal opening (Fig. 12D). An accessory foramen for the hyo- mandibular branch of the trigeminal trunk also occurs in Hypophthalmus (Hypophthalmidae) (Howes, 1983:22) and has been reported in Pimelodidae by Lundberg et al. (1990). We have also found this condition in a specimen of Chrysichthys cranchii (Bagridae). The presence of an additional foramen in the prootic is considered to be a derived feature. In Belodontichthys as in other Siluridae the optic and trigeminal foramina are widely separated by a bony wall, but in Hemisilurus the separation is membraneous. The occipital roof is somewhat flattened in Belodontichthys, Ompok, Wallago and Silurus but in Hemisilurus and Kryptopterus it is pinnacle-shaped. In all these taxa apart from Silurus, the epi- and supraoccipital semicircular canal tubes are elevated to form a high, posterior ridge to the cranial roof whereas in Silurus the canals are indicated only as slight ridges along the posterolateral margins of those bones. In all siluroids the exoccipital forms the low
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