Figure 130.— Range of diversity in 



body form and external features of 



representative aracanid genera: 



Strophiurichthya inermis (above) and 



S. ro6u«(u«— in front of each, above, 



nasal region as seen externally (far 



left) and the olfactory lamellae as 



seen with the top of the nasal sac 



removed, and. below, 



outline of cross section of middle of body. 



Exoccipital. — Cartilage filled along all of its edges 

 of articulation with the other cranial bones; articulates 

 through cartilage posteromedially with its opposite 

 member and posterodorsally with the supraoccipital; 

 articulates through cartilage and extensive inter- 

 digitation posterolaterally with the epiotic, ventro- 

 medially with the basioccipital, and anteroventrally with 

 the prootic, while ventroiaterally it articulates mainly 

 through cartilage but with some interdigitation with the 

 pterotic. The lateral surface of the extreme posterior end 

 of the exoccipital is overlain by and interdigitated with 

 the neural spines of the rudimentary first two ab- 

 dominal vertebrae, as explained in the section on the 

 vertebral column. In this region the exoccipitals form the 

 lateral wails of the foramen magnum, while the dorsal 

 wall is formed by the cartilaginous sheet between the 

 posteromedial edges of the two exoccipitals; the ventral 

 surface of the foramen magnum is bounded by the basi- 

 occipital. 



Supraoccipital. — Laterally expanded and with a 

 well-developed supraoccipital crest directed posteriorly; 

 cartilage filled along all of its edges of articulation with 

 the other cranial bones; articulates by extensive inter- 

 digitation anteriorly and anterolaterally with the fron- 

 tals, while it articulates through cartilage laterally with 

 the epiotics and ventrally with the exoccipitals. 



Otic Region. 



Pterotic. — Cartilage filled along all of its edges of 

 articulation with the other cranial bones; articulates 

 through cartilage and interdigitation anteroventrally 

 with the prootic, posteroventrally with the exoccipital, 

 posterodorsally with the epiotic, and anterodorsally with 

 the sphenotic; the posterodorsal portion of the sphenotic 

 intervening between the pterotic and frontal so that the 



latter two bones do not articulate directly with one 

 another. For a short distance medially along its antero- 

 ventral edge the pterotic supports through cartilage the 

 posterodorsal edge of the hyomandibular. The antero- 

 lateral region of the pterotic is prolonged ventrally into a 

 stout shaft whose medial edge articulates through fi- 

 brous tissue with the posterodorsal region of the hyoman- 

 dibular while laterally it is broadly overlain and exten- 

 sively interdigitated with the posttemporal and medially 

 it helps to support by fibrous tissue the expanded lateral 

 end of the ossified Baudelot's ligament. 



Sphenotic. — Cartilage filled along all of its edges of 

 articulation with the other cranial bones; articulates 

 through cartilage and interdigitation posterodorsally 

 with the pterotic and, to a lesser extent, with the epiotic, 

 while anterodorsally it interdigitates with the frontal; 

 medially in the posterior wall of the orbit the sphenotic 

 articulates through cartilage and interdigitation with the 

 pterosphenoid, prootic, and pterotic. The dorsolateral 

 surface of the sphenotic is broadly overlain by the fron- 

 tal. 



Epiotic. — A rounded sturdy cone anteriorly but 

 prolonged posteroventrally into a thin flange; cartilage 

 filled along all of its edges of articulation with the other 

 cranial bones; articulates through cartilage and inter- 

 digitation dorsally with the frontal, anteroventrally 

 along its lateral edge with the sphenotic and pterotic and 

 posteroventrally with the exoccipital and medially, 

 through cartilage only, with the supraoccipital. 



Prootic. —Cartilage filled along all of its edges of 

 articulation with the other cranial bones, except an- 

 teriorly and ventrally where it articulates with the para- 

 sphenoid; articulates by interdigitation ventromedially 

 with the laterally compressed keellike dorsal region of 



