. Bulletin of the British Museum (Natural History). Zoology . 2 8 P. H. GREENWOOD The whole subject of cranial and dental morphology in the Lake Victoria Haplo- chromis flock is dealt with in greater detail below (pp. 56-99). For the moment it must suffice to note that although species showing specialized dental, pharyngeal and syncranial characters are very distinctive (and readily advertise their feeding habits) the majority are linked, through species with these characters at an intermediate stage, to the generalized type (see Text-figs 4, 5 and 65-69). Many of these morpho-lin
. Bulletin of the British Museum (Natural History). Zoology . 2 8 P. H. GREENWOOD The whole subject of cranial and dental morphology in the Lake Victoria Haplo- chromis flock is dealt with in greater detail below (pp. 56-99). For the moment it must suffice to note that although species showing specialized dental, pharyngeal and syncranial characters are very distinctive (and readily advertise their feeding habits) the majority are linked, through species with these characters at an intermediate stage, to the generalized type (see Text-figs 4, 5 and 65-69). Many of these morpho-lineages appear also to be truly phyletic ones because the component taxa can be related, primarily, though shared specializations, and secon- darily by degrees of specialization in the same characters. There are a few instances where intraspecific variability in a complex of specialized characters is such that, were only the extreme individuals known, they would probably be classified as a separate genus (for example, H. xenognathus, Greenwood, 1957 ; and H. welcommei, Greenwood, 1966b). Surprisingly, amongst species with such diverse feeding habits, there is little diversity in the shape, length or number of gill rakers; the modal numbers of rakers are 9 or 10. The plankton-feeding species ( H. erythrocephalus ; Text-fig. 6) have. Fig. 6. Outer row of gill rakers on the first gill arch (left) of A : H. brownae (an insecti- vorous-piscivorous species) and B : H. erythrocephalus (a phytoplankton feeder). rakers that are but marginally longer than those of an insectivore, are only a little more closely spaced, and are more numerous by one or two rakers. No explanation can be offered, except that the feeding mechanism of phytoplankton feeders may not require the evolution of close-set and fine rakers. These species entangle the plank- ton in mucus boli that are too large to pass through even the relatively wide spaces between rakers of the first row ; subsequent rows of rakers i
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