L. TRIGONUS • O 

 R. TRIQUETER ▼ V 

 R. BICAUDALIS ■ □ 



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The fundamental plan of the skull is even more con- 

 servative in the ostraciids than in the aracanids, and 

 nearly all of the differences between the species to be 

 pointed out involve the vertebral column. 



In all of the species of the Indo-Pacific genera (Lac- 

 toria, Tetrosomus, Ostracion, and Rhynchostracion) a 

 small but distinct myodome is present, while in all of the 

 species of the Atlantic genera (Acanthostracion, Rhine- 

 somus, and Lactophrys) the myodome is essentially ab- 

 sent. In the Atlantic species there is no anterodorsal roof 

 to the prootic at the appropriate place over the posterior 

 eye muscles, although in the rear of the orbit where the 

 prootics are in contact medially the two bones are curved 

 upward to form a shallow concavity bounded posteriorly 

 by a low crest, which region can be considered as the pos- 

 terior and posterodorsal walls of a rudimentary 

 myodome. If these surfaces of the prootics in the Atlan- 

 tic species were a little more concave and the posterior 

 myodome region more uplifted and anterodorsally 

 oriented, a small myodome could be said to be present, 

 but such is not quite the case. In the Indo-Pacific species 

 a small and shallow myodome is more distinctly present. 

 In all of the species of the more generalized family 

 Aracanidae, a small but distinct myodome is present. 



In all of the Indo-Pacific species at least two of the 

 vertebrae posterior to the last which helps support the 

 last dorsal fin basal pterygiophore have trifid neural 

 spines, while none of the neural spines are trifid or even 

 bifid in the Atlantic species. The greatest number of 

 trifid neural spines is found in Lactoria cornuta, in which 

 the 14th to 17th vertebrae have them. In L. fomasinii 

 there is one less, the trifid spines being present on the 

 14th to 16th vertebrae. In Tetrosomus concatenatus and 

 T. gibbosus well-developed trifid neural spines are pres- 

 ent on the 15th and 16th vertebrae, and a less strongly 

 developed one may be present or absent on the 17th 

 vertebra. In Ostracion lentiginosum they are trifid on the 

 15th to 17th vertebrae, as they also are in 0. tuber- 

 culatus and Rhynchostracion rhinorhynchus, except that 

 the spine of the 17th vertebrae is sometimes single rather 

 than trifid. The trifid neural spines apparently afford a 

 broader surface of support for the carapace in this region 

 than does the single neural spine. 



In all of the Atlantic species and in the Indo-Pacific 

 Lactoria fornasinii the first five vertebrae are involved in 

 the fusion complex with the base of the skull, while all of 

 the Indo-Pacific species except L. fornasinii have only 

 the first four vertebrae involved in the fusion complex. Of 

 the two species of Ostracion examined, at least one has 

 slightly less fusion between the first four vertebrae than 

 in the other Indo-Pacific species. In Ostracion tuber- 

 culatus the first two vertebrae are fully fused to one 



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NUMBER OF TEETH 



Figure 165.— Number of teeth in upper (solid 

 symboU) and lower (open symbols) jaws in relation 

 to sUndard length, to show the slightly greater 

 number of teeth in the upper versus the lower jaw, 

 and the relatively negligible increase in number of 

 teeth with increasing standard length at sizes 

 greater than about 40 mm SL; Lactophry» trigonut 

 Rhinesomiu triqueter, R. bicaudalit. 



