basal pterygiophi 



Figure »2.—Monacanthu» ciliatus: dorsal (left) 

 and ventral (right) views of skull, 51.3 mm SL, Colombia. 



Brachaluteres and Psilocephalus, in which only the first 

 spine remains. The single spine of Brachaluteres is rela- 

 tively well developed, but that of Psilocephalus is very 

 short and weak, as is its pterygiophore. In most genera 

 the first dorsal spine is ornamented with downward and 

 backward projecting spiny processes, especially along the 

 two posterolateral edges, while the anterior surface is 

 usually ornamented with smaller and less regularly ar- 

 ranged barbs, these oriented either upward, downward, 

 or straight forward. However, a few genera have the 

 barbs on the anterior face of the spine as well developed, 

 or almost so, as on the posterolateral edges: Laputa, 

 Chaetoderma, Arotrolepis, and Acanthaluteres . Others 

 have a relatively low and plain ornamentation over most 

 of the surface of the spine which is not greatly rougher 



than that of the skin: Amanses, Cantherhines, and 

 Alutera. In a few genera (Brachaluteres and Pseuda- 

 luteres) there are low barbs mainly on the anterior face, 

 these continued onto the posterior face only distally. In 

 two genera, Paraluteres and Psilocephalus, there are no 

 barbs at all. In Paraluteres the barbs have obviously 

 been lost in conjunction with the role of this filefish as a 

 mimic of the pufferfish Canthigaster valentini, the spine 

 being covered by smooth scaleless skin and probably not 

 fully erectile in life. The smoothness of the spine of Psilo- 

 cephalus is undoubtedly related to the reduction of the 

 spine in that genus. 



The second dorsal spine is always much smaller than 

 the first and is absent in Brachaluteres and Psiloceph- 

 alus. The second spine always has a sturdy rounded basal 



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