. Bulletin of the British Museum (Natural History). 46 P. H. GREENWOOD graphically, would form the inferior hyohyoid merge with the fibres forming the body of the muscle I have identified as the interhyoideus. Thus, the situation in Gymnarchus has all the appearance of a secondary return to the primitive condition in which the constrictor hyoideus ventralis has not split into inter- and hyohyoid portions. That the condition is not strictly a primitive one is indicated by the separate left and right inter- cum-hyohyoideus muscles. The poorly developed hyohyoideus of Gymnarchus contrasts markedl
. Bulletin of the British Museum (Natural History). 46 P. H. GREENWOOD graphically, would form the inferior hyohyoid merge with the fibres forming the body of the muscle I have identified as the interhyoideus. Thus, the situation in Gymnarchus has all the appearance of a secondary return to the primitive condition in which the constrictor hyoideus ventralis has not split into inter- and hyohyoid portions. That the condition is not strictly a primitive one is indicated by the separate left and right inter- cum-hyohyoideus muscles. The poorly developed hyohyoideus of Gymnarchus contrasts markedly with the hypertrophy of that muscle in all mormyrids. Gymnarchtts also differs in having the branchiostegal rays (and their associated musculature) free from the ventral body muscles. Nevertheless, the branchiostegal membrane ( the interray muscles and tendons) still does not provide a ventrolateral floor to the branchial cavity. This is formed by a membranous sheet which runs, on each side, from the inner face of the corresponding ceratohyal to the upper part of the sternohyoid muscle. In effect, each branchiostegal membrane constitutes a lateral half pouch (opening medially but blind anteriorly) over the sternohyoid and hypaxial muscles. The intermandi- bularis muscle covers these pouches and their medial openings. This greater pouch is open posteriorly across the breadth of the body. Identifying the hyoid muscles of Gymnarchus is further complicated by the presence of a pair of small muscles that lie immediately anterior to the sternohyoid. Fig. 21. Gymnarchus nilolious: Sternohyoid and ceratohyal-urohyal muscles in left lateral view; Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original British Museum (Natural History). London : BM(NH)
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