20 ME. G. A. BOTJLENGEE ON A COLLECTION 



Petrochromis, g. n. 



Body moderately elongate; scales ctenoid. Jaws with very broad bands of minute, 

 bristle-like teeth, with incurved bi- or tricuspid crowns; maxillary concealed under the 

 prgeorbital. Dorsal with 17 or 18 spines, anal with 3. Vertebrae 32 (16+16). 



Distinguished from Tilapia by the very broad bands of minute teeth with incurved 

 crowns, from Simochromis by the absence of conical teeth in the prsemaxillary. 



21. Petrochromis polyodon, sp. n. (Plate VI. fig. 1.) 



Crowns of teeth brown. Depth of body 1\ to 2f times in total length, length of head 

 2f to 3 times. Snout with convex upper profile, 1J to If diameter of eye, which is 4 

 to 41 times in length of head and 11 in interorbital width ; mouth hardly extending 

 to below anterior border of eye ; 4 or 5 series of scales on the cheek ; large scales on 

 the opercle. Gill-rakers very short, 12 or 1 3 on lower part of anterior arch. Dorsal 

 XVII-XVIII 8-9 ; spines increasing in length to the sixth or seventh, which measures 

 about I length of head ; longest soft rays f to § length of head. Pectoral nearly as 

 long as head. Ventral reaching vent or origin of anal. Anal III 7-8 ; third spine 

 longest, as long as longest dorsal. Caudal truncate. Caudal peduncle as long as deep. 

 Scales 32-34 ^3 ; lat. 1. ffj^. Olive-brown, whitish beneath ; fins grey or blackish. 



Total length 135 millim. 



Two specimens from Kinyamkolo, and two from Mbity Eocks. 



The prsemaxillary and mandibular bones are very massive, and the maxillary is much 

 reduced in size ; the ascending processes of the prsemaxillaries extend to between the 

 anterior borders of the orbits and are received in a deep excavation, to which the strong 

 occipital crest extends ; the parietal crests are produced forwards as far as the frontals ; 

 the prseorbital is large, and the chain of suborbitals very slender. Only the first rib is 

 absolutely sessile, the following being attached to the back of the transverse processes 

 at a short distance from the centre ; the epipleurals extend to the twelfth rib ; the last 

 two prsecaudal vertebras form a hsemal bridge. 



Perissodus, g. n. 



Body elongate ; scales cycloid. Teeth rather large, unequal in size, few, with swollen 

 bases and low slightly -notched crowns, compressed transversely to the axis of the jaws, 

 disposed in a single series; maxillary exposed. Dorsal with 18 spines, anal with 3. 

 Vertebras 35 (17 + 18). 



The extraordinary dentition which characterizes this new genus may be looked upon 

 as an extreme specialization of that exhibited by Tilapia, a specialization in an 

 opposite direction from that attained by Petrochromis. 



