sphenoid. The posterior half of the basioccipital is 

 hidden from view by the forward displacement of the 

 highly modified first five abdominal vertebrae, as ex- 

 plained in the section on the vertebral column. 



Exoccipital. — Cartilage filled along all of its edges 

 of articulation with the other cranial bones; articulates 

 by interdigitation posterodorsally with the epiotic, ven- 

 trolaterally with the pterotic, ventromedially with the 

 basioccipital, and anteroventrally with the prootic. 

 Posteriorly the exoccipitals form the lateral walls and 

 most of the dorsal walls of the foramen magnum, while 

 ventrally the foramen is closed by the dorsal surface of 

 the basioccipital. Posterodorsally the medial edges of the 

 exoccipitals closely approach, but do not meet, one 

 another and are held together by a thin sheet of fibrous 

 tissue. More anteriorly, however, the dorsomedial edges 

 of the exoccipitals are more distantly separated and the 

 space between them is filled by a thin sheet of cartilage. 

 The lateral surface of the extreme posterior end of the 

 exoccipital is overlain by and interdigitated with the 

 more anterior of the highly modified first five abdominal 

 vertebrae, as explained in the section on the vertebral 

 column. 



Supraoccipital. — Laterally expanded; no supraoc- 

 cipital crest; cartilage filled along all of its edges of ar- 

 ticulation with the other cranial bones; articulates by 

 interdigitation anteriorly and anterolaterally with the 

 frontals and posterolaterally with the epiotics. 



Otic Region. 



Pterotic. — Cartilage filled along all of its edges of 

 articulation with the other cranial bones; articulates by 

 interdigitation posterodorsally with the epiotic, antero- 

 dorsally with the sphenotic, anteroventrally with the 

 prootic and posteroventrally with the exoccipital. For a 

 short distance medially along its anteroventral edge the 

 pterotic supports by fibrous tissue the posterodorsal edge 

 of the hyomandibular. The anterolateral region of the 

 pterotic is prolonged ventrally into a stout shaft that is 

 mostly obscured from lateral view by the broadly over- 

 lying posttemporal, to which it is interdigitated. At the 

 ventromedial end of this shaft the pterotic helps to sup- 

 port by fibrous tissue the laterally expanded end of the 

 ossified Baudelot's ligament and medially the postero- 

 lateral edge of the hyomandibular. 



Sphenotic. —Cartilage filled along all of its edges of 

 articulation with the other cranial bones; articulates by 

 interdigitation posterodorsally with the frontal, postero- 

 ventrally with the pterotic and for a short distance with 

 the anterodorsal end of the posttemporal, dorsomedially 

 with the pterosphenoid, and ventromedially with the 

 prootic. The dorsolateral surface of the sphenotic is 

 broadly overlain by the frontal. 



Epiotic. — More or less rounded, but with a short 

 ventrally directed process from its ventrolateral edge; 



cartilage filled along all of its edges of articulation with 

 the other cranial bones; articulates by interdigitation 

 dorsally with the frontal, anteroventrally along its lateral 

 edge with the sphenotic and pterotic, posteroventrally 

 with the exoccipital, and medially with the supraoc- 

 cipital. For a short distance the medial edge of the 

 epiotic is in contact with the cartilaginous plate that 

 separates the medial edges of the exoccipitals. 



Prootic. — Cartilage filled along all of its edges of 

 articulation with the other cranial bones, except at its 

 anteriormost region of articulation with the para- 

 sphenoid; articulates by interdigitation ventromedially 

 with the laterally compressed keellike dorsal region of 

 the parasphenoid, ventrolaterally with the pterotic, pos- 

 teroventrally with the basioccipital and exoccipital, dor- 

 somedially with the pterosphenoid and dorsolaterally 

 with the sphenotic. Along most of the anterior edge of its 

 laterally expanded posterior portion the prootic possesses 

 a slight concavity with which the dorsal head of the hyo- 

 mandibular articulates by fibrous tissue. The antero- 

 medial region of the prootic possesses a long forward 

 extension under the orbit which makes contact by inter- 

 digitation with the parasphenoid at the level of the 

 prefrontals. A bony myodome is essentially absent. 



Orbital Region. 



Frontal. — Wide posterolaterally; extremely thin 

 and delicate anterolaterally, where it is in close contact 

 with the ventral surface of the cuirass; articulates by in- 

 terdigitation posteriorly with the supraoccipital and 

 posterolaterally with the epiotic and sphenotic, broadly 

 overlying the latter. Anteroventrally, in the orbital 

 region, the frontal interdigitates with the ptero- 

 sphenoid. Anteriorly the frontal is prolonged into a very 

 thin projection that overlies the dorsomedial edge of the 

 prefrontal and the dorsal surface of the ethmoid cartilage 

 and reaches almost to the posterior end of the ethmoid. 

 Since the thin anterolateral expansion of the frontal is 

 closely held by fibrous tissue to the ventral surface of the 

 cuirass over the orbital region, a large portion of the fron- 

 tal is very easily torn away when the cuirass is removed. 

 Thus, the few figures of the skulls of trunkfishes that 

 have been previously published usually show the frontal 

 incompletely and unrealistically with ragged edges. 



Prefrontal. — Large and wedge-shaped; cartilage 

 filled along its medial edge where it is continuous with 

 the ethmoid cartilage; articulates by fibrous tissue dor- 

 somedially with the frontal, while ventrally it articu- 

 lates by fibrous tissue and, in large specimens, by slight 

 interdigitation with the parasphenoid. Its major surface 

 of articulation, however, remains with the ethmoid car- 

 tilage that broadly intervenes between the prefrontal and 

 the ethmoid. 



Parasphenoid. — Elongate, running almost the 

 entire length of the skull; expanded ventrally along near- 

 ly all of its length into a thin keel, which itself is laterally 



