Family Diodontidae 



Comparative diagnosis (contrast with that of the 

 Tetraodontidae). — Teeth incorporated into the matrix 

 of the biting edge of the jaws as small and more or less 

 rounded units; premaxillaries and dentaries fully fused 

 to their opposite members in the midline; lateral surface 

 of the maxillary with a deeply indented or laterally flang- 

 ed surface, the thickened ridges increasing its strength; 

 the jaws massive; a large trituration plate always present 

 in the upper and lower jaws; first and second pharyngo- 

 branchials with minute teeth, third pharyngobranchial 

 with minute teeth or toothless, and sometimes absent; 

 dorsal and ventral hypohyal usually both present; inter- 

 hyal never present; anterior edge of ectopterygoid rela- 

 tively straight and only slightly concave; ethmoid and 

 vomer greatly reduced in size and fused together into a 

 relatively thin and functionless plate; palatine not 

 notched posteriorly, but broadly sutured thereto, and 

 taking its main support from, the frontal; prefrontal 

 small or absent; frontals exceptionally wide and massive; 

 anterior end of parasphenoid exceptionally wide and 

 deeply concave, the concavity not being a place of ar- 

 ticulation for other bones; rear margin of the orbit form- 

 ed by the frontal alone; sphenotic with a long, slender, 

 laterally directed sturdy prong from its anterolateral 

 edge; frontal broadly in contact posteroventrally in the 

 rear of the orbit with the prootic, and, to a lesser extent, 

 with the pterosphenoid, but not in contact with the 

 sphenotic; suboperculum with a prominent anteriorly 

 directed prong attached by a short ligament to the pos- 

 terior end of the interoperculum, the latter not articu- 

 lating with the operculum; supracleithrum positioned 

 horizontally almost in the same line as the axis of the 

 body; postcleithrum a single short piece, no longer than 

 about the distance along the scapula to the lowest ac- 

 tinost; supraoccipital crest dorsoventrally compressed 

 and entirely in a horizontal plane, wider than deep 

 throughout its length; exoccipital condyles poorly 

 developed; all of the vertebrae anterior to the first basal 

 pterygiophore of the dorsal fin and a few of those poste- 

 rior to the last basal pterygiophore of the dorsal fin with 

 bifid divergent neural spines; many of the more poste- 

 rior abdominal vertebrae and several of the more ante- 

 rior caudal vertebrae with prominent lateral flanges from 

 the ventrolateral surfaces of the centra; neural spines of 

 the vertebrae supporting the basal pterygiophores of the 

 dorsal fin short and broad, not slender shafts and not 

 penetrating deeply the interspaces between the pteryg- 

 iophores; a supraneural element never present; none of 

 the basal pterygiophores of the dorsal and anal fins inter- 

 digitated with one another; none of the abdominal 

 vertebrae with haemal arches, complete or incomplete; 



several of the vertebrae posterior to the bases of the last 

 basal pterygiophores of the dorsal and anal fins antero- 

 posteriorly compressed, much shorter in centrum length 

 than those more anteriorly; abdominal vertebrae always 

 greater in number than the caudal vertebrae; dorsal and 

 anal fins more posterior in position; only two vertebrae 

 fully posterior to that whose haemal spine is the last sup- 

 port of the last anal fin basal pterygiophore; caudal fin 

 supporting skeleton with no free epural, no free hypurals, 

 and no free parhypural, and with the haemal spine of the 

 penultimate vertebra fused to its centrum; a prominent 

 lateral flange present on the fused hypural-centrum 

 plate; haemal canal not penetrating the last vertebral 

 complex; caudal fin rays modally either 9 or 10, in the 

 ventral region of the fin only the single lowermost ray un- 

 branched, perhaps 11 caudal rays in the Eocene 

 Prodiodon; scales always relative massive, whether long 

 erectile quills or shorter spines borne on a large triradiate 

 plate, or some combination of erectile and fixed spines. 



Detailed description of Diodon holocanthus. 



Material examined: — Five cleared and stained 

 specimens, 12.3-113 mm; one dry partially disarticu- 

 lated skeleton, 124 mm. 



SKULL. 



Occipital Region. 



Basioccipital. — A short column, expanded antero- 

 laterally; cartilage filled at its anterior and anterolateral 

 edges; articulates by interdigitation posterolaterally with 

 the exoccipitals, anterolaterally with the prootics, and 

 anteriorly with the overlying parasphenoid. In all of the 

 specimens examined a certain amount of externally visi- 

 ble cartilage persists at the region of junction of the 

 basioccipital, exoccipitals, and prootics, as it also does at 

 the junction of the exoccipital, prootic, and pterotic and 

 at the junction of the posterior portions of the para- 

 sphenoid and prootics. Extremely large specimens can be 

 expected to have more extensive interdigitation and less 

 cartilage visible in these areas than shown in the illus- 

 trations. The rim of the round concave posterior end of 

 the basioccipital articulates by fibrous tissue with the 

 rim of the concave anterior face of the first vertebra. The 

 posterodorsal surface of the basioccipital forms the lower 

 wall of the foramen magnum. 



Exoccipital. — Cartilage filled at all of its edges of 

 articulation with the other cranial bones; articulates by 

 interdigitation anterodorsally with the epiotic, laterally 

 with the pterotic, anteroventrally with the prootic, and 



Figure 279.— Range of diversity in body form 



in the Diodontidae: Diodon holocanthus (left) 



and Chilomycterua achoepfi (right). 



343 



