5201 O a 

 • D. 

 DC. 

 ■ c. 



VC 



▼ c, 

 Oc 



holocanthus 



Figure 301. — Chitomycterus mauretanicus: 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, 92.2 mm SL. 



C. orbicularis, which has a much different skull shape 

 and arrangement of the trituration teeth. All four spe- 

 cies have gill rakers along the anterior edge of the fourth 

 arch, as does C. orbicularis (as well as Diodon tind 

 Dicotylichthys), all of these species being from the Indo- 

 Pacific with the exception of C. affinis, from the Atlan- 

 tic, while the other species of Chilomycterus {anten- 

 natus, antillarum, mauretanicus, schoepfi, and 

 spinosus), all from the Atlantic, do not have gill rakers 

 along the anterior edge of the fourth arch. The only 

 species of Chilomycterus with teeth on the fifth 

 ceratobranchial are orbicularis and three {affinis, 

 reticulatus, tigrinus) of the four species with a pitted cup 

 nasal apparatus, no spines on the forehead and 10 caudal 

 rays. 



The evidence indicates that C. affinis, atinga, reticu- 

 latus, and tigrinus are all closely related, and that C. 

 antennatus, antillarum, mauretanicus, schoepfi, and 

 spinosus are a distinct subgroup of equally closely related 

 species. 



The Indo-Pacific C. orbicularis would seem to have its 

 closest relatives among the primarily Indo-Pacific former 

 group than to the latter Atlantic group, for it shares a few 

 differential characters (prefrontal absent, gill rakers 

 present on anterior edge of fourth arch, teeth on fifth cer- 

 atobranchial) with them and none with the Atlantic 

 group. Most of the differential features of C. affinis, 

 atinga, reticulatus, and tigrinus are specializations 

 (pitted open cup nostril, loss of prefrontal and pharyn- 

 gobranchial of third arch, usual presence of teeth on the 

 fifth ceratobranchial, and usual loss of the dorsal hypo- 

 hyal), and their retention of 10 caudal fin rays is the only 

 way in which they are more generalized than C. orbicu- 

 laris and the other speciose subgroup of Chilomycterus 

 (and other diodontids). The presence in C. orbicularis of 

 a toothless small pharyngobranchial of the third arch, 

 two hypohyals, and a nasal apparatus of a tube with two 

 nostrils is less specialized than in C. affinis, atinga, retic- 

 ulatus, and tigrinus, but its other features that distin- 

 guish it from them are specialized (narrowness of the 

 region around the palatine and anterior end of the fron- 

 tal, numerous rows of trituration teeth, nine caudal fin 

 rays), while it shares with them the specialized features 

 of the loss of the prefrontal and the presence of teeth on 

 the fifth ceratobranchial. 



The exclusively Atlantic subgroup of Chilomycterus 

 is specialized only in the loss of gill rakers on the anterior 



10 15 20 2! 



Number of tritufation tooth plates i 



Figure 302.— Chart showing the increased 



number of trituration plates in the upper 



jaw with increasing specimen size, with 



the same relationship holding for the lower 



jaw also, in Diodon holocanthus, D. hyslrix, 



Chilomycterus schoepfi, C. mauretanicus, 

 C. atinga, C. antillarum, and C. reticulatus. 



