. Bulletin of the British Museum (Natural History). 22 E. TREWAVAS The ethmo-vomerine relationship is ilhistrated in figs 13 and 14. The vomer surrounds the rostral end of the ethmoid cartilage and laterally makes a sutural union with the lateral ethmoid at the articular surface with the upper arm of the head of the palatine. In most cichlids (and other Perciformes) the mesethmoid ossification meets the vomer dorsally between these two points, forming a complete or nearly complete bony roof for the ethmoid cartilage. This is true of Pelmato- chromis, Chromidotilapia and the genera of the Haplo
. Bulletin of the British Museum (Natural History). 22 E. TREWAVAS The ethmo-vomerine relationship is ilhistrated in figs 13 and 14. The vomer surrounds the rostral end of the ethmoid cartilage and laterally makes a sutural union with the lateral ethmoid at the articular surface with the upper arm of the head of the palatine. In most cichlids (and other Perciformes) the mesethmoid ossification meets the vomer dorsally between these two points, forming a complete or nearly complete bony roof for the ethmoid cartilage. This is true of Pelmato- chromis, Chromidotilapia and the genera of the Haplochromis-gToup and also of Trislramella and most species of Tilapia, in which there is a small area of cartilage left free between right and left ethmo-vomerine sutures (fig. 13). But in T. rendalli (3 specimens tested) the anterior edge of the mesethmoid is rounded and free from the vomer. It was the contrast between T. rendalli on the one hand and T. zillii and T. bnsumana on the other that led Regan (1920) to recognize the subgenus Coptodon for T. zillii and T. bnsumana. Regan did not then know that the ethmo- vomerine sutures characterize also the type species of Tilapia, T. spanmanii A. Smith. We do not now consider it possible to separate T. zillii and T. rendalli sub- generically. Although the ethmo-vomerine union cannot be used as an absolutely diagnostic feature of Tilapia, its opposite, the freedom of these bones from each other, appears to be a constant feature of Sarotherodon, verified by me in at least one specimen each of fourteen species. Sarotherodon is specialized in this respect as also in its dentition, especially the pharyngeal, and in its reproductive arrangements. One may suggest a possible functional explanation for the failure of the meseth- moid to meet the vomer in Sarotherodon. These predominantly microphagous species are characterized by a very broad head, providing a long and broad bucco- pharynx for the passage of large quantities of food-laden wate
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