. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 510 BuUctin Museum of Coniparafivc Zoology. Vol. 144, Xo. 8 m-odermal. preural centrum hypural centrum parhsrpural , 5 mm Figure 25. lotabrycon proecox. Lateral view of caudal skeleton ( $ rays. Note enlarged lower procurrent caudal and antorbital in Bryconamericus and Astyanax. Phenacobrycon has infra- orbitals, the shape and size of each similar to the corresponding elements in Brycon- americus and Astyonax (Fig. 13). Landonia has but four separate infraorbitals (Fig. 15). The first separate element ( =
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 510 BuUctin Museum of Coniparafivc Zoology. Vol. 144, Xo. 8 m-odermal. preural centrum hypural centrum parhsrpural , 5 mm Figure 25. lotabrycon proecox. Lateral view of caudal skeleton ( $ rays. Note enlarged lower procurrent caudal and antorbital in Bryconamericus and Astyanax. Phenacobrycon has infra- orbitals, the shape and size of each similar to the corresponding elements in Brycon- americus and Astyonax (Fig. 13). Landonia has but four separate infraorbitals (Fig. 15). The first separate element ( = fused first and second infraorbitals?) is elongate; its leading edge is indented, thus per- mitting the flared distal end of maxillary bone to lie unco\'ered by it when mouth is closed. The second separate element ( = fused third and fourth?) is slightly enlarged compared to corresponding ele- ments in Phenacoljrycon and Bryconameri- cus. The number and relative sizes of the circumorbitals is the same in all five speci- mens of Landonia. lotabrycon has only three circumorbitals (Fig. 11), the number and relative size the same in the three specimens examined. The opercle, subopercle, interopercle, and preopercle are similar in size and shape in all three genera and are much as in Bryconamericus and Astyanax. In Phenacobrycon and Landonia the dorsal- most part of the preopercular branch of the laterosensor\' canal system is enclosed in a separate, tubular, bony supra- preopercle. Visceral arches (Figs. 17-21). The x'isceral arches are similar in all tliree genera, the principal difference lying in the numbers of gill rakers. All have a well-ossified fourth basibranchial, thus agreeing with Bryconamericus and Astya- nax but differing from Brycon, which has only three ossified basibranchials (exclud- ing "accessor\'" ossifications). Pectoral girdle (Figs. 22, ). The pectoral girdles are virtually identical in Phenacobrycon and Landonia. The dis-. Please note that these
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