to the skin. In P. prionurus the skin is smooth to the 

 touch, except on the caudal peduncle of mature males 

 where there are two pairs of retrorse barbs and a patch of 

 short upright setae extending forward to the level of the 

 anal fin origin. The fact that there are very low weakly 

 developed spinulose scales on the head and ventral sur- 

 face from the mouth to the anus is only readily apparent 

 in cleared and stained specimens. 



The scales of monacanthids vary greatly in the degree 

 and structure of the spinulation and in the size and 

 degree of overlap (if any) of the usually rounded to rec- 

 tilinear basal plates, but they are always much smaller 

 than in balistids. In most species the basal plates more or 

 less broadly but irregularly overlap, while in others (e.g., 

 Chaetoderma) there is little if any overlapping of the 

 relatively large and more or less triangular plates. The 

 scales of Chaetoderma are more or less regularly ar- 

 ranged in rows, and, according to Fraser-Brunner 

 (1941b: 178), they are in distinct longitudinal tracts in 



Figure 125.— From left to right: 



A, Paraluteres prionurus. ventral view of 



skull, 46.4 mm SL, Seychelles; B. Pervagor 



apilosomus, ventral view of skull, 



ca. 65 mm SL, Hawaii; C, Psilocephalus 



barbatu^, dorsal and ventral views 



of skull, 137 mm SL, Singapore. 



Arotrolepis, while in the great majority of monacanthids 

 the arrangement is irregular. There is always at least one 

 upright spinule (least developed in Paraluteres, best in 

 Chaetoderma) per scale plate, and often many. The in- 

 dividual scale plates are largest in Chaetoderma, in 

 which many of the upright spinules support large der- 

 mal flaps, and smallest in Psilocephalus. 



All monacanthids, like balistids, have two nostrils in a 

 scaleless area on both sides, each nostril usually at the 

 end of a short tube, but sometimes more or less flush with 

 the surface (especially Paraluteres and Psilocephalus). 



