Figure 101.— External features of other 

 representative monacanthid genera: 

 Brachaluteres trossulus — upper left, nasal 

 region as seen externally (olfactory lamella. 

 if present, indistinct in specimen examined): 

 lower left, scales from upper middle region 



of body, including a lateral line canal 



bearing scale; no encasing scales at end of 



pelvis (or below it). 



The size and shape of the basal pterygiophore sup- 

 porting the spiny dorsal fin are highly diverse among the 

 monacanthids. It is expectedly least developed in Psilo- 

 cephalus, in which only a single, short, thin rudi- 

 mentary spine is present. The basal pterygiophore of 

 Psilocephalus is a thin triangular bone (as seen in lateral 

 view) whose dorsal edge is only slightly expanded lateral- 

 ly and whose ventral edge is held by fibrous tissue only 

 anteriorly to the dorsal surface of the supraoccipital. The 

 dorsal surface of the pterygiophore at the place of articu- 

 lation of the spine is only slightly expanded laterally and 

 upraised to support the equally only slightly laterally ex- 

 panded base of the spine, the whole mechanism being far 

 less complex than in other monacanthids. 



In all other monacanthids the basal pterygiophore is 

 relatively much larger and sturdier than in 

 Psilocephalus. Most typically, as illustrated for Mona- 

 canthus ciliatus, it is a large bone whose flattened or 

 gently curved ventral surface is closely applied by fibrous 

 tissue and sometimes interdigitation to the dorsal sur- 

 face of the skull from the level of the front or middle of 

 the eye posteriorly, obscuring much of the supraoc- 

 cipital from view. It is laterally expanded in about the 

 middle of its length into two thick buttresses supporting 

 the articular facets of the base of the first spine, while 

 posteriorly an upraised medial buttress supports the sec- 

 ond spine, posterior to which the basal pterygiophore 

 becomes a thick vertical plate with a laterally expanded, 

 slightly concave, dorsal edge. Anterior to the large lateral 

 expansion supporting the first spine, the pterygiophore 

 rapidly narrows into a thinner vertical plate whose dor- 

 sal edge is slightly expanded laterally and either flatten- 

 ed or convex. The region of the pterygiophore posterior to 

 the second spine is supported mainly by the epiotics, 

 while the middle region is supported by the supraoc- 

 cipital, and, especially in the case of the buttresses for 

 the first spine, by the frontals. Anterior to the first spine, 

 the laterally compressed portion of the pterygiophore is 

 supported by the frontals alone in most genera, but by 

 the supraoccipital as well in those three genera (Mona- 

 canthus, Stephanolepis, Paramonacanthus) in which 



that bone extends far forward above the orbit as a low 

 vertical crest (the possible phylogenetic significance of 

 which is discussed under the section on generic relation- 

 ships). 



The above applies well to most of the genera in which 

 the first dorsal spine is above or behind the middle of the 

 eye, with the basal pterygiophore extending from behind 

 the skull to about the level of the front of the eye. In 

 Paraluteres, with the dorsal spine distinctly in front of 

 the eye, the basal pterygiophore is enormously elongate, 

 reaching nearly the entire length of the skull, although it 

 retains the same basic makeup as in more normal mona- 

 canthids. The basal pterygiophore in Paraluteres articu- 

 lates posteriorly with the supraoccipital, which it broadly 

 overlies, and along the ventral edge of its laterally com- 

 pressed plate with the epiotics, while the middle two- 

 thirds of its length is firmly held to the frontals and its 

 anterior end to the ethmoid. Many of the specializations 

 of Paraluteres are associated with its mimetic rela- 

 tionship with a Canthlgaster pufferfish (Tyler 1966). 



The basal pterygiophore in the especially deep-bodied 

 Chaetoderma is very high and domelike, while in the 

 species ofAlutera it is slightly smaller than in most other 

 monacanthids, having only a short anterior vertical crest 

 in front of the level of the first spine, and the pterygio- 

 phore is not as broadly held ventrally to the supraoc- 

 cipital and epiotics. 



In two genera the basal pterygiophore is of more or less 

 normal size in lateral surface area, but much thinner 

 than in other monacanthids {Psilocephalus excepted). In 

 the deep-bodied Brachaluteres the pterygiophore sup- 

 porting the single but well-developed dorsal spine is es- 

 pecially high, but it is much thinner and less heavy than 

 normal, the buttresses for the spine being only slightly 

 expanded laterally and most of the surface of the ptery- 

 giophore being a thin compressed plate. In Paraluteres, 

 in which the two skin covered dorsal spines are probably 

 not fully erectible in life, the pterygiophore is of reduced 

 sturdiness and of relatively short length, although the 

 buttressing for the spines is more or less normal. 



In comparison to the spiny dorsal fin, there is even 



