28 U.S. NATIONAL MUSEUM BULLETE^ 2 84 



tube are formed by the exoccipital and epiotic ventrally and the 

 epiotic alone dorsally. 



Ventroposteriorly, each exoccipital forms a coneshaped condyle, 

 which is joined to the centrmn of the first vertebra. On the ventral 

 external surface of each exoccipital slightly anterior to the condyle, 

 there is an opening to a canal (vagal foramen) that passes through 

 the exoccipital and enters the cranial cavity just ventral to the large 

 concavity discussed above. The external opening of the canal is 

 shielded anteriorly by a ventrally raised portion of the exoccipital. 



The exoccipitals contribute the major portion of the walls sur- 

 rounding the foramen magnum. The remainder of the wall, the mid- 

 floor, is formed by the basioccipital. 



Intercalar (figs. 1, 2, 4, 5). — Each intercalar is a thin, irregularly 

 shaped bone with a median ridge externally. Each bone overlaps its 

 respective prootic, exoccipital, and pterotic, and portions of the 

 intercalar's irregular margin may insert into shallow pockets in these 

 three bones. A broad ligament, continuous on the pterotic spine, 

 extends from the ridge of each intercalar to the anterodorsal surface 

 of its respective cleithrum. A short ligament from the area where 

 the ridge of the intercalar meets that of the pterotic spine extends 

 to the anterior end of the ventral arm of the posttemporal. 



SuPRAOCciPiTAL (figs. 2, 3, 5). — The supraoccipital forms the 

 dorsoposterior cap of the skull. The dorsoposterior convex portion 

 (concave on its internal surface) of the supraoccipital joins each 

 exoccipital and epiotic along the dorsomesial margins of those bones. 

 Thin laminar extensions of the convex portion overlap the epiotics 

 externally (thus covering the epiotic-supraoccipital joint). A narrow, 

 median, dorso-anterior raised extension of the supraoccipital convex 

 portion forms the posterior border of the large dorsomedian opening 

 to the supratemporal canal of the laterosensory system. The lateral 

 margins of the raised extension contact the parietal crest in the 

 region of the dorsomedian opening. Anterior to the raised extension 

 is a broad notch that allows passage of the supratemporal canal. 

 Anterior to the notch the supraoccipital is raised again, as a bladeUke 

 extension. The posterior edge of the blade forms the median anterior 

 margin of the dorsomedian opening to the supratemporal canal. 

 VentraUy the blade expands laterally on either side as a thin flange. 

 The dorsomesial portions of each parietal and the dorsoposterior 

 portions of each frontal rest on top of the flange on its respective side. 



CiRCUMORBiTAL BONES (figs. 1, 3, 7). — There are five circumorbital 

 bones on each side in Entomacrodus. The anteriormost (lachrymal) 

 is much larger than the others. The lachrymal is broad dorsally 

 where it is tightly joined to the ventrolateral surface of the lateral 

 ethmoid. Anteriorly, the lachrymal is deep and concave. The end of 



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