Family Aracanidae 



Comparative diagnosis (contrast with that of the 

 Ostraciidae). — Supraorbital region of the skull relative- 

 ly strong, the frontal relatively thick in this region and 

 not held extraordinarily firmly to the undersurface of the 

 carapace; frontal with a long forward extension broadly 

 overlying the dorsolateral surface of the ethmoid to, or 

 very nearly to, the extreme anterior end of the snout; 

 supraoccipital with a long, thin, vertical crest extending 

 posteriorly well beyond the level of the base of the skull; 

 epiotic much prolonged posterolaterally, well beyond the 

 level of the base of the skull, and beyond the level of the 

 distal end of the supraoccipital crest; prootic shelf with a 

 recurved wing from its lateral edge which makes contact 

 with the anterior edge of the laterally expanded pos- 

 terior region of the bone; myodome small but distinct; 

 four pharyngobranchials, the first a toothless suspen- 

 sory element, the second either toothless or with minute 

 teeth, the third and fourth with few but large teeth; 

 branchiostegals usually 2 -t- 4, but sometimes 1 -I- 4 or 2 

 -I- 3; ethmoid relatively deep and platelike and not much 

 laterally expanded anteriorly; pterosphenoid and para- 

 sphenoid with a broad region of suturing in the interor- 

 bital septum; parasphenoid anterior to its region of 

 suturing with the anteromedial edges of the prootic 

 shelves either not much expanded laterally or expanded 

 laterally only for a portion of its length there, the lateral- 

 ly expanded portion being either anteriorly near the 

 vomer or posteriorly near the prootic shelf, but not form- 

 ing a complete hard palate over the roof of the oral 

 cavity; posterodorsal region of the coracoid with a 

 moderately to well-developed posterior prong; two small 

 vertebrae, represented mostly by neural arch material, at 

 the front of the vertebral column variously fused and 

 sutured to one another and to the exoccipitals and 

 basioccipital, the fusion region without a ventral process 

 below the level of the centra; vertebrae immediately fol- 

 lowing the fusion region with long neural spines and 

 anteroposteriorly much compressed centra with well- 

 developed transverse processes; the centra of the first 

 several vertebrae anterior to the caudal plate not much 

 compressed anteroposteriorly; haemal arch and spine of 

 the penultimate vertebra relatively well developed and 

 autogenous; caudal fin rays i, 9, i (except the Eocene 

 Proaracana, with i, 8, i); postcleithrum (especially the 

 ventral piece) greatly expanded posteriorly into a deep 

 thin plate reaching the level of the anterodorsal ends of 

 the more anterior anal fin basal pterygiophores; supra- 

 neural long and well developed, with a relatively deep 

 ventral flange making contact with most or all of the 

 neural spines of the predorsal vertebrae (exclusive of 

 those of the small first two fused vertebrae), extending 

 anteriorly well beyond the level of the anteroventral end 

 of the first dorsal fin basal pterygiophore; haemal spines 

 on the more posterior abdominal and on most of the 

 caudal vertebrae well developed, especially those above 

 the anal fin basal pterygiophores which they support 

 through a connective tissue sheet rather than by close ap- 

 position; first anal fin basal pterygiophore without 



Figure 129.— Typical body form in 

 the Recent Aracanidae: Aracana aurita. 



anterolateral wings from its distal end; haemal canal en- 

 tirely straight, in the midline under the vertebral centra 

 and relatively well enclosed by well-developed haemal 

 arches and spines on all but the more anterior ab- 

 dominal vertebrae; none of the vertebrae posterior to the 

 anteriormost two fused vertebrae sutured to one another; 

 uppermost pectoral fin ray relatively well developed, 

 consisting of two halves with a few cross-striations dis- 

 tally, the two halves of about the same length or one half, 

 usually the lateral, only slightly shorter than the other; 

 body relatively deep, the distance between the distal 

 ends of the first dorsal and anal fin basal pterygiophores 

 being contained less than 3 times in the SL and the dis- 

 tance between the top of the rear of the cranium and the 

 ventralmost edge of the pectoral girdle being contained 

 about one and three-fourths to slightly more than twice 

 in the SL; carapace open behind the dorsal and anal fins; 

 carapace with a well to only slightly developed ventral 

 keel; caudal peduncle always with scale plates isolated 

 from the carapace proper, often almost completely, if not 

 completely, encircling it. 



Detailed Description of Kentrocapros aculeatus. 



Material examined.— Two cleared and stained 

 specimens; 90.0-90.7 mm. 



Occipital Region. 



Basioccipital. — A short column, slightly expanded 

 anterolaterally; cartilage filled anterodorsally; ar- 

 ticulates through cartilage and interdigitation dorsally 

 with the exoccipitals and anterolaterally with the 

 prootics, while its ventromedial surface is broadly over- 

 lain by and extensively interdigitated with the bifurcate 

 posterior end of the parasphenoid. The upper region of 

 the extreme posterolateral surface of the basioccipital is 

 more or less indistinguishably fused with the basal region 

 of the rudimentary first two abdominal vertebrae, as ex- 

 plained in the section on the vertebral column. 



