Figure 50.— Balistapug undulatusi upper 



left, scale plates of the tympanum region 



just behind the gill slit and above the 



pectoral fin base; upper middle, scales from 



upper middle region of body, including 



lateral line canal bearing scales; 



upper right, ventral view of encasing scales 



at end of pelvis (anterior to left), with 



arrow indicating major region of flexibility 



(two terminal branches of modified fin-r>y 



element protrude posteriorly); 



lower left, nasal region as seen externally (above) 



and the olfactory lamellae as seen with 



the top of the nasal sac removed. 



Detailed description of Balistapus undulatua. 



Material examined.— Two cleared and stained 

 specimens, 122-124 mm, and three wet partially disar- 

 ticulated skeletons of approximately the same size as the 

 two previous specimens, prepared by maceration. 



Occipital Region. 



Basioccipital. —A short column, expanded antero- 

 dorsally; cartilage filled at its anterior and dorsal edges; 

 articulates by interdigitation dorsally with the exoc- 

 cipitals, anterolaterally with the prootics, and antero- 

 medially with the overlying posterior end of the para- 

 sphenoid. The rim of the round concave posterior end of 

 the basioccipital articulates by fibrous tissue with the 

 concave anterior face of the centrum of the first vertebra. 

 The ventromedial surface of the basioccipital is deeply 

 concave throughout its length. This concave channel is 

 mostly hidden from view by the overlying parasphenoid, 

 but the channel opens to the exterior posteriorly at the 

 base of the posterior bifurcation of the parasphenoid. 

 Anteriorly the channel opens into the myodome where 

 the anterior end of the basioccipital forms the postero- 

 lateral and posterodorsal walls of the myodome. 



Exoccipital.— Cartilage filled at its ventral edge; 

 articulates by interdigitation dorsally with the epiotic, 

 laterally with the pterotic and ventromedially with the 

 basioccipital. Posteriorly the exoccipitals form the 

 lateral and ventral walls of the foramen magnum, while 

 dorsally the foramen is closed by the thin piece of car- 

 tilage between the dorsomedial edges of the two exoc- 

 cipitals. The posterior surface of the exoccipitals firmly 

 articulates by fibrous tissue and slight interdigitation 

 with the anterior surface of the bifid neural spine of the 

 first vertebra. The posteromedial portion of the exoc- 

 cipital, which forms the lateral wall of the myodome and 

 the firmest place of attachment of the neural spine of the 



first vertebra, is stout and thickened, probably repre- 

 senting the much modified region of the exoccipital con- 

 dyle. 



Supraoccipital. — Wide posteriorly, tapering to a 

 point anteriorly; expanded dorsomedially throughout its 

 length into a low crest; cartilage filled at its edges of ar- 

 ticulation with the other cranial bones; articulates by in- 

 terdigitation anterolaterally with the slightly overlying 

 frontals and posterolaterally with the epiotics. The 

 posterodorsal end of the supraoccipital is expanded into 

 a thickened articular face with a medial concavity to 

 support the anteroventral shaft of the first basal 

 pterygiophore of the spiny dorsal fin. This medial con- 

 cavity is partially bordered by the epiotics. The antero- 

 ventral shaft of the pterygial element is held by tough 

 fibrous tissue to both the supraoccipital and the epiotics. 



Otic Region. 



Pterotic. — Cartilage filled along all of its edges of 

 articulation with the other cranial bones; articulates by 

 interdigitation dorsomedially with the epiotic, antero- 

 laterally with the sphenotic, anteroventrally with the 

 prootic, and anteromedially on its ventral surface with 

 the basioccipital. More or less transversely along its ven- 

 tral surface, the pterotic is produced into a sturdy flange 

 whose depth is greatly increased in about the middle of 

 its length. The anterior surface of this stout and elongate 

 portion of the ventral flange of the pterotic articulates by 

 fibrous tissue with the posterodorsal edge of the hyoman- 

 dibular. Just anterior to its ventral flange, the pterotic 

 bears a slight depression which continues anteriorly over 

 the prootic and into which the dorsal head of the hyo- 

 mandibular is held by fibrous tissue. In about the middle 

 of its lateral surface, the pterotic bears a deep vertical 

 groove into which the posttemporal fits and is held im- 

 movably by extensive interdigitation. 



