phore just posterior to that supporting the first spine. 

 The second spine also has a very short and blunt pos- 

 teromedial expansion which is accommodated in a low 

 concavity on the pterygiophore just behind the medial 

 flange over which the base of the spine rotates. The lock- 

 ing mechanism of the two spines is basically similar to 

 that in the Balistidae and continuous positions of erec- 

 tion from partial to full are possible. 



The ventral surface of the basal pterygiophore is 

 strongly interdigitated with the top of the skull, especial- 

 ly with the frontals, and extends forward to the level of 

 the prefrontals. In addition to the frontals, the basal 

 pterygiophore is interdigitated with the supraoccipital 

 and epiotics. The pterygiophore is especially sturdy and 

 laterally expanded in the region below the first dorsal 

 spine. 



Fin rays and pterygiophores. — There are usually 

 between 30 and 36 fin rays; all of the rays unbranched, 

 but with cross-striations. In the illustrated specimen the 

 31 fin rays are borne on an equal number of basal 

 pterygiophores. Each fin ray has a small, unpaired, distal 

 pterygiophore between the bifurcate base of the ray. 

 Each basal pterygiophore, except for the last several, has 

 a well-developed lateral ridge along its length, except at 

 the very proximal and distal ends of the element, for 

 muscle attachment. The lateral ridges decrease slightly 

 in height posteriorly in the series. Distally the ridge is es- 

 pecially laterally expanded and curved slightly ventrally 

 to form a hooklike process. The first basal pterygiophore 

 is slightly the largest in the series, the other pterygio- 

 phores decreasing slightly in length posteriorly in the 

 series. The first basal pterygiophore articulates by 

 fibrous tissue between the neural spines of the fourth and 

 fifth abdominal vertebrae, while the last few basal 

 pterygiophores are between those of the 9th and 10th 

 caudal vertebrae. Posterodorsally the first basal 

 pterygiophore interdigitates with the second basal 

 pterygiophore, and all of the subsequent basal pterygio- 

 phores are extensively interdigitated to one another 

 along their edges of contact, except for the last few basal 

 pterygiophores which are held to one another mainly by 

 fibrous tissue instead of extensive interdigitation. Just 

 below their distal ends the successive pterygiophores are 

 not in close contact and a foramen is formed between 

 their apposed edges. The degree of interdigitation 

 between the pterygiophores decreases slightly in the 

 series posteriorly, as does their degree of contact with the 

 neural spines supporting them. The basal pterygial 

 elements are cartilage filled at their dorsal and ventral 

 edges. 



Fin rays and pterygiophores. — There are usually 

 between 30 and 35 fin rays supported by an equal 

 number of basal pterygiophores or by only one or two less 

 than the number of rays; all of the rays unbranched but 

 with cross-striations. In the illustrated specimen the 31 

 fin rays are supported by 29 basal pterygiophores. Each 



fin ray has a small, unpaired, distal pterygiophore 

 between its bifurcate base. The basal pterygiophores of 

 the anal fin are similar to those of the soft dorsal fin and 

 bear the same kind of lateral flanges for muscle at- 

 tachment along most of their lengths. The first two basal 

 pterygiophores of the anal fin articulate by fibrous tissue 

 between the haemal spines of the first and second caudal 

 vertebrae, while they articulate by extensive interdigi- 

 tation with one another, as do the other pterygiophores in 

 the series. This interdigitation decreases posteriorly in 

 the series to the extent that the last few pterygiophores 

 articulate to one another mainly by fibrous tissue. The 

 pterygiophores decrease slightly in length posteriorly in 

 the series, as does the height of their lateral flanges. 

 There are usually three to four anal fin basal pterygio- 

 phores articulated between successive haemal spines, 

 whereas in the dorsal fin there are usually only two or 

 three basal pterygiophores between successive neural 

 spines, this because the number of basal pterygiophores 

 in the dorsal and anal fins is similar, whereas the dorsal 

 fin is longer based and extends over more vertebrae than 

 does the anal fin. 



Anatomical diversity. — The monacanthids are one of 

 the more diversified families of plectognaths, com- 

 parable in many ways to the degree of diversity found in 

 the Recent triacanthodids, although without as much 

 diversity in dentition and with no indication of two major 

 phyletic lines within the family such as is evident in the 

 Recent triacanthodids. Fossil species of monacanthids 

 are not yet known, although the ancestral balistids have 

 been found as early as the Oligocene. The mona- 

 canthids, with about 90 species, are one of the most 

 speciose families of plectognaths, only the tetraodontids 

 having more species, perhaps a few over 100. Even 

 though two subfamilies of tetraodontids (tetraodontins 

 and canthigasterins) are easily recognizable and none 

 such are in the monacanthids, the latter still are 

 probably more anatomically diversified than the former. 

 That is, there is no sharp break in the structural con- 

 tinuity of monacanthids, while there is in tetraodontids 

 between Canthigaster and the tetraodontins, only par- 

 tially bridged by Carinotetraodon. 



Most monacanthids have an only slightly elongate 

 body form, similar to that of balistids, although they 

 tend to be thinner bodied (more laterally compressed) 

 than balistids. However, monacanthids range from those 

 of nearly perfectly rounded outline {Brachaluteres, 

 Chaetoderma), in which the greatest body depth, in- 

 cluding the normal pelvic region expansion, approaches 

 the standard length, to the extremely elongate Psiloce- 

 phaluti, whose depth is contained many times in the 

 standard length. 



Although always over the top of the head, the place- 

 ment of the first dorsal spine is variable, ranging from 

 over the rear edge of the eye (many genera) to well 

 forward of the eye (Pseudaluteres only), while in most 

 genera it originates somewhere over the rear half of the 

 eye. Two dorsal spines are present in all genera except 



