Figure 29S.—Chilomycterus schoepfi: cross 



section of upper (above in each set) and 



lower jaws just to one side of the midline, 



showing the numbers of dental units in 



the biting edges of the jaws and in the 



trituration plates, increasing greatly 



in number with increasing specimen size: 



A, 97.6 mm SL and B, 166 mm SL. 



the two species of Dicotylichthys examined, both had 9 

 caudal vertebrae, one with 11 abdominal and the other 

 with 12. 



The uppermost pectoral fin ray in diodontids is usually 

 relatively short and formed of two approximately equal 

 unsegmented halves, but in the single specimen of Chi- 

 lomycterus affinis examined, a large adult of 310 mm, 

 this ray is a single piece of bone, the two halves that can 

 be expected to be found in smaller specimens apparently 

 having completely fused, or one of the halves having been 

 resorbed. 



Generic relationships. — It is difficult to discuss ge- 

 neric relationships without first establishing a natural 

 division of the family into genera, and as pointed out in 

 the preceding section, the two most recent revisions 

 recognized three noncomparable genera on the basis of 

 nostril and spine characteristics, with each of the revi- 

 sions having serious limitations. The osteology of the spe- 

 cies examined here helps to some extent, but leaves 

 much left to be done. 



Figure 299.—ChUomycterug antillarum: cross 



section of upper (above) and lower jaws just 



to one side of the midline, showing the numbers of 



dental units in the biting edges of the jaws 



and in the trituration plates, 116 mm SL. 



Figure 300.— Chilomycterus atinga: cross 

 section of upper (above) and lower jaws just 

 one side of the midline, showing the numbers 



of dental units in the biting edges of the 

 jaws and in the trituration plates, 173 mm SL. 



Within Chilomycterus, by far the most speciose genus 

 of diodontids, three groups are recognizable. Chilomyc- 

 terus affinis, atinga, reticulatus, and tigrinus are unique 

 among the diodontids by having the third pharyngo- 

 branchial absent, a pitted open cup nasal apparatus (at 

 least as adults), no spines on the forehead and 10 caudal 

 fin rays. Three of these four species are also unique by 

 the presence of only a single hypohyal. All four species 

 have the prefrontal absent and similar skull shapes, the 

 prefrontal otherwise being absent in diodontids only in 



