. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. posttemporal â ceratobranchials Figure 21. Landonia latidens. Dorsal view of right half of hyoid (minus interhyal) and gill arches (branchiostegal rays removed) ( soecimen). In all three genera the lateral foramen of the exoccipital is umisiially large. Jaws (Figs. 5, 6, 11-16). 'The jaws of Fhcnacohnjcon (Figs. 5, 13, 14) are similar to the jaws of Bnjconamericus and Astya- nax. The dorsal surface of the distal end of the premaxillary is notched to receive the ventral edge of the maxillary's ascend- ing proces
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. posttemporal â ceratobranchials Figure 21. Landonia latidens. Dorsal view of right half of hyoid (minus interhyal) and gill arches (branchiostegal rays removed) ( soecimen). In all three genera the lateral foramen of the exoccipital is umisiially large. Jaws (Figs. 5, 6, 11-16). 'The jaws of Fhcnacohnjcon (Figs. 5, 13, 14) are similar to the jaws of Bnjconamericus and Astya- nax. The dorsal surface of the distal end of the premaxillary is notched to receive the ventral edge of the maxillary's ascend- ing process. This notch is a relatively constant feature in many Tetragonopterinae and other characids with multicuspid teeth. It is present in such generalized fonns as Brycon, Sahninus, and the African genus Alestes. In these forms the lateral wall of the notch in the premaxillaiy bone lies external to the ascending maxillar\- (as in Fig. 13 of Phenacobrycon). Landonia the notch is absent, and entire ascending limb of the maxillary lies external to the premaxillary. This can supracleithrum. imb In the be seen in Figure 15, a lateral view of Landonia, but it is more apparent when the jawbones are viewed from above (not figured). A second marked peculiarity' of the maxillary in Landonia is the shape of its Figure 22. lolabrycon proecox. Internal view of left half of pectoral girdle ( specimen). distal end, the trailing edge of which is curved outwards. From manipulation of the alizarin preparations the impression is gained that these peculiarities of the maxillary in Landonia are functionally significant in aligning the straight-edged teeth of the maxillary with those on the dentary close enough that their shearing or scissor-like action can be effected. This action possibly could be further enhanced by dilation of the rami of the lower jaws; in Landonia, as in Phenacobrycon and many other characins, the two halves of the lower jaw are movably joined at the symphysis by a symphys
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