20 MR. G. A. BOULENGER ON A COLLECTION 
PETROCHROMIS, g. 0. 
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 
preorbital. Dorsal with 17 or 18 spines, anal with 3. Vertebre 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 premaxillary. 
21. PrrRocHROMIS POLYODON, sp. n. (Plate VI. fig. 1.) 
Crowns of teeth brown. Depth of body 23 to 22 times in total length, length of head 
23 to 3 times. Snout with convex upper profile, 14 to 13 diameter of eye, which is 4 
to 43 times in length of head and 1} 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 13 on lower part of anterior arch. Dorsal 
XVII-XVIII 8-9 ; spines increasing in length to the sixth or seventh, which measures 
about 2 length of head; longest soft rays 3 to 3 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 82-34 =e lat. 1. = Olive-brown, whitish beneath ; fins grey or blackish. 
Total length 135 millim, 
Two specimens from Kinyamkolo, and two from Mbity Rocks. 
The premaxillary and mandibular bones are very massive, and the maxillary is much 
reduced in size; the ascending processes of the premaxillaries 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 preorbital 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 precaudal vertebre form a hemal bridge. 
PERISSODUS, g. 1. 
Body elongate; scalescycloid. ‘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. 
Vertebre 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. 
