1902.] PLECTOGXATHOUS FISHES. 289 



short rough spine at the end of the pelvis. Most of the prre- 

 cauclal interneurals co-ossified to form a bony trough, attached to 

 the skull, and receiving the retracted dorsal spines. In other 

 characters similar to the two preceding families. 



Genera. 



1. Balistes Linn.^ — Body compressed, caudal peduncle short. 

 Scales moderate or large, juxtaposed, osseous. Jaws usually even 

 in fi"ont. Gill-openings behind the eyes. Dorsal usually mth 

 3 spines — the first strong and just behind the eye, the second 

 locking it when erected, the third, if present, remote from them. 

 Soft dorsal with 23-35 rays. Anal with 20-30. Caudal rounded 

 or truncate, the outer rays often more or less produced. Pelvis 

 projecting. Ventrals represented by a short, rough, movable spine. 

 The movable pelvis, abdominal sac, and vential flap are very 

 similar to those of Triodon, but much less developed. Palatine 

 T-shaped, the cross-piece articulating with ethmoid and maxillary, 

 the vertical limb with the ectopterygoid. Vertebras 18. 



2. MoxACANTHUS Cuv. — Differs from Balistes in that the scales 

 are smaller ; the palatine is a straight rod attached to maxillary 

 and ethmoid, having lost the lower vertical limb which in Balistes 

 articulates with the ectopterygoid ; the third dorsal spine is 

 always absent and the caudal always rounded. The first dorsal 

 spine is above the eye, and if it has distinct barbs these are 

 usually arranged in two series. The gill-openings are often below 

 the posterior part of the eye. There are 18 vertebrae. 



In this genus are included all those species of 3fonacanthus, as 

 understood by Gunther, with a movable ventral spine. 



The transition is perfect from those with a rough dorsal spine 

 without distinct barbs, to those with a series of minute barbs on 

 each side, and so to those with barbs strongly developed. 



M. penicilligerus Cuv. belongs to this genus, the ventral spine 

 being movable, and the barbs on the dorsal spine exactly similar 

 to those of the closely-allied M. tomentosus, although their regular 

 arrangement is obscured by the well-developed fleshy filaments. 

 In the development of the ventral sac and flap, some species of 

 this genus almost rival Triodoii. 



3. Paraluteres Bleeker. — Difiers from Monacanthus in that 

 the single dorsal spine is weak, not fully erectile, and the ventral 

 spine is absent, or small and fixed. 



4. Pseudaluteres Bleeker. — Differs from Monacanthus in that 

 the dorsal spine is in advance of the orbit, the ventral spine is 

 absent, and the pelvis is entirely concealed. 



5. Pseudomonacanthus Bleeker. — Diflfers from Monacanthus in 

 that the ventral spine is immovable, ankylosed to the pelvis, and 



1 The Oligocene genus Acanthoderma Agassiz is scarcely distinguishable fi'om 

 Salistes. 



Proc. Zool. Soc— 1902, Yol. II. No. XIX. 19 



