460 cichlim:. 



unsafe guide for the determination of species, and yet, in the absence of other characters, 

 I have felt compelled to maintain the old generic divisions, after modifying their 

 definitions. Of the genera here dealt with as having representatives in the Nile system 

 it is necessary to observe that Hemichromis passes completely into Paratilapia, which 

 leads, almost without gaps, to Pelmatochromis on the one hand, to Haplochromis on the 

 other, the latter passing into Tilapia^ which again merges into Petrochromis. These 

 generic divisions are unsatisfactory, but they are the best I can suggest at present. 

 The number of anal spines, as a generic character, has proved of even less importance 

 than I previously supposed. 



As observed above, the greatest difficulties are encountered in dealing with the 

 Cichlids of Lake Victoria. It would seem as if the forms inhabiting that great lake 

 had sprung from a small number of original types which had been isolated in its waters 

 and had been modified into a multitude of species according to lines of evolution different 

 from those followed by other colonies, such as the remarkable set of Lake Tanganyika, 

 and contrary to the rules laid down in ichthyological systems. This view is borne out 

 by the striking resemblance in scale- and fin-formulas and even in markings of most of 

 the species of Lake Victoria and also by the fact that two or three only of these species 

 are identical with or very closely related to forms occurring in neighbouring rivers. 



The characters of the seven genera here dealt with may be thus contrasted : — 



I. Maxillary bone exposed at the end when the mouth is closed ; outer teeth miicuspid, at least 



in the adult. 

 Teeth in one, two, or rarely three series in each jaw ; scales cycloid . 1. Hemichromis, Peters. 

 Teeth in three or more series in each jaw ; scales usually ctenoid ; no 



very prominent papillose pad on each side o£ the pharynx ... 2. Paratilapia, Blkr. 

 Teeth in two or more series in each jaw ; scales ctenoid ; a very 



prominent papillose pad on each side o£ the pharynx, strongly 



projecting in front of the upper branch of the first branchial 



arch 3. Pelmatochromis, Stdr. 



II. A very small part of the maxillary exposed when the mouth is 



closed ; jaws with very broad bands of small conical teeth, the 



band in the upper jaw horseshoe-shaped 4. Platytamiodus, Blgr. 



III. Maxillary bone exposed at the end when the mouth is closed ; 



teeth uni-, bi- or tricuspid ; scales ctenoid 5. Haplochromis, Hilg. 



IV. Maxillary bone entirely or nearly entirely concealed when the mouth is closed ; teeth bi- or 

 tricuspid. 



Outer teeth bicuspid 6. Tilapia, A. Smith. 



All the teeth tricuspid, forming very broad bands 7. Petrochromis, Blgr. 



Of these seven genera three only are represented in the Nile proper. 



