ed, only 2 had a free upper hypural. Other variation in 

 the monacanthid caudal skeleton (discussed in Tyler 

 1970b) is that the usually horizontal crest for muscle at- 

 tachment on the last centrum is reduced to a prong in 

 Brachaluteres trossulus, and its neural and haemal 

 canals are better developed than in other monacanthids, 

 being roofed over distally rather than open. In 

 Psilocephalus only the middle 6 to 8 rays of the ex- 

 tremely elongate caudal fin are branched, rather than 

 having 10 branched rays as in all other monacanthids. 

 The posterior edge of the fused hypural plate is much 

 more convex in Psilocephalus than in all other monacan- 

 thids, while in Brachaluteres the cleft in the posterior 

 edge of the fused hypural plate, that marks the region of 

 fusion between what in generalized plectognaths like 

 triacanthodids are the second and third hypurals, is 

 much deeper than in all other monacanthids. 



In balistids and in all but two genera of monacanthids 

 the side of the rear part of the cranium just above the 

 region of articulation of the upper end of the hyoman- 

 dibular is formed by the sphenotic anteriorly and by the 

 pterotic posteriorly, the hyomandibular articulating with 

 a groove across the surface of the prootic and pterotic. 

 The sphenotic usually has a well -developed (least 

 developed in Rudarius) ventrally or anteroventrally 

 directed prong from its lower edge and the pterotic a 

 larger ventrally directed flange that overlies the postero- 

 dorsal end of the hyomandibular. The anterolateral edge 

 of the sphenotic forms the lower rear margin of the orbit, 

 with the frontal forming the margin more dorsally. 



The two exceptions to these conditions are Oxymona- 

 canthus and Pseudaluteres, in both of which the 

 sphenotic is displaced posteriorly by a ventral extension 

 of the frontal, the side of the rear of the cranium above 

 the articulation of the hyomandibular being formed by 

 the frontal anteriorly and the pterotic posteriorly, with 

 the sphenotic squeezed in between. The sphenotic 

 retains a short ventrally directed prong and the pterotic a 

 normally developed large flange overlying the postero- 

 dorsal end of the hyomandibular, which articulates in a 

 normal manner with a groove across the surface of the 

 prootic and pterotic. The frontal, which forms all of the 

 posterior margin of the orbit in these two genera to the 

 exclusion of the sphenotic, has a ventrally directed prong 

 from its anteroventral end in this region. This prong on 

 the frontal is comparable to that on the sphenotic of all 

 other monacanthids, where it forms the lower rear mar- 

 gin of the orbit. 



Monacanthids always have the prootic shelf much less 

 strongly developed than in balistids. The shelf in 

 monacanthids does not extend as far forward as the level 

 of the middle of the orbit, except in Psilocephalus, in 

 which the shelf is long, thin, and delicate and reaches to 

 the front of the orbit. Within these limits, the prootic 

 shelf of monacanthids is variously developed, from 

 relatively well to only moderately, and in two genera it is 

 completely absent, these being Oxymonacanthus and 

 Pseudaluteres. 



All monacanthids have an outer series of three and an 

 inner series of two teeth in each premaxillary, the inner 



series closely applied to the inner (or under) surface of 

 the outer series and to the bone. In the dentary there is a 

 single series of teeth, corresponding to the outer series of 

 the upper jaw, usually three in number, but reduced to 

 two in Psilocephalus barbatus, Rudarius ercodes and R. 

 minutus, Brachaluteres trossulus and (according to 

 Clark and Gohar 1953:46) B. baueri and (according to 

 Scott 1969:40) B. wolfei, Paraluteres prionurus, and Oxy- 

 monacanthus longirostris (presumedly also only two 

 teeth in the closely related 0. halli Marshall as well). 

 While three teeth are present in the dentary of Pseuda- 

 luteres nasicornis, the outermost tooth is much smaller 

 than in other monacanthids with three dentary teeth. In 

 all monacanthids the medial edges of articulation of the 

 apposed dentaries are well-denticulated. 



The teeth are relatively thinner and have more 

 delicate cutting edges than in balistids, but they are 

 usually notched in basically the same way in both 

 families. The least notched condition of the teeth among 

 the monacanthids examined is found in Acanthaluteres 

 spilomelanurus, in which the two most medial teeth in 

 both the upper and lower jaws (the most medial tooth of 

 both premaxillaries and both dentaries) are relatively 

 straight edged, forming a broad relatively sharp straight 

 nipping edge along the front of the mouth. The position 

 of the mouth tends to be terminal or only slightly supra- 

 terminal in most monacanthids, but it is distinctly 

 supraterminal in the elongate Psilocephalus barbatus. 

 The mouth of Oxymonacanthus is relatively small and 

 laterally compressed, with the teeth also relatively nar- 

 row but elongate, for what is probably a specialized mode 

 of feeding between the branches of coral of this strictly 

 reef-dwelling species. Among the monacanthids examin- 

 ed, the teeth are most massive in Amanses scopas, but 

 this could in large part be a function of the large size of 

 the single specimen examined. It will be of interest to 

 know the size of the teeth in juveniles and young adults 

 of Amanses, which have yet to be collected (Randall 

 1964:335). 



While the number of vertebrae in the ancestral 

 balistids is almost invariably 7 -(- 11 = 18 (in contrast to 

 their ancestral triacanthoids in which it is 8 -I- 12 = 20), 

 the number of vertebrae in monacanthids is highly 

 variable, although the most frequent formula is 7 H- 12 = 

 19 and none of the species examined normally has as few 

 as the 18 vertebrae of balistids. Only a few species of 

 monacanthids have other than seven abdominal verte- 

 brae, most of the variation in total number being in the 

 caudal series. The only monacanthids normally with six 

 abdominal vertebrae are the two Atlantic species of 

 Monacanthus {ciliatus and tuckeri, the subgenus Lep- 

 rogaster of Fraser-Brunner, with 6 -I- 13 = 19 in contrast 

 to the 7 -I- 12 = 19 of the Indo-Pacific species) and 

 Acanthaluteres spilomelanurus (6 -I- 14 = 20). The only 

 monacanthids normally with eight abdominal vertebrae 

 are Oxymonacanthus longirostris (8 -(- 17 = 25) and 

 Pseudaluteres nasicornis (8 -I- 18 = 26). 



When the number of vertebrae is increased beyond 

 what is probably the generalized number of 19, it is 

 usually by the addition of one or more vertebrae in the 



173 



