12 ILLINOIS BIOLOGICAL MONOGRAPHS [182 



caudad sliglitly dorsad and laterad on the epiotic ridge, whicli in this 

 position projects over and forms a part of the roof of the temporal 

 fossa. At the posterior end of the process there is a foramen, passing 

 cephalad into the bone for one of the lateral line canals. The process 

 forms a ridge and extends antero-mesad, uniting with a similar ridge 

 from the opposite side in the median line. The dorsal surface of the 

 ridge is porous and possesses several striations parallel with the ridge. 

 At the anterior end the bone joins by suture with the frontal; it is 

 supported laterally by the pterotic and mesially and ventrally by the 

 epiotic and supra-occipital. In the suture between the parietal and 

 supra-occipital on each side of the median line is a foramen for the 

 ramus lateralis accessorius vessels. The inner surface of the bone lies 

 largely on the dorsal surface of the supra-occipital, but lateral to the 

 plate-like supra-occipital, which covers the greater part of the cranial 

 cavity; the parietal forms a part of the dorsal surface of the cavity 

 where it arches over the space between supra-occipital and pterotic. 



Opisthotic. — The opisthotic (Fig. 6, opo) is a small plate-like 

 bone, oval in outline, lying between the pterotic and the exoccipital. 

 The antero-dorsal edge overlaps the postero-ventral edge of the pterotic 

 and the ventral edge overlaps the exoccipital, thus forming the latero- 

 caudal edge of the skull. It is drawn out caudad into a rounded process 

 which projects beyond the exoccipital and even with the pterotic. The 

 opisthotic forms the posterior lateral wall of the temporal fossa, from 

 the point of its union with the pterotic on the mesial surface. The 

 posterior end of the bone has a bulb-like appearance and is united by a 

 cartilaginous ligament to the suprascapular. 



Exoccipital. — The exoccipitals (Figs. 1, 6, 8, eo) are irregu- 

 lar bones which form a part of the lateral and of the posterior surface 

 of the cranium and of the brain case. A well defined angle, ventral to 

 the opisthotic, separates the lateral from the candal surfaces. That 

 part which forms the porterior portion lies directly below the 

 epiotic and a part of it forms the base of the epiotic ridge. The dorsal 

 edge also borders on and forms the posterior edge of the floor of the 

 temporal fossa. That part posterior to the angle extends backward 

 and forms a rounded head, the occipital condyle, which looks caudad 

 and slightly ventrad, articulating with an anterior process of the first 

 vertebra. The two condyles are dorso-laterad to the basioccipital and 

 laterad to the foramen magnum. Ventrad to the epiotic ridge and the 

 temporal fossa, the bone extends caudad and mesad, luiiting in the 

 median line with its fellow of the opposite side ; thus forming the roof 

 of the medulla oblongata. It is overlapped in the median line by a 

 ventro-posterior process of the supra-occipital. Slightly above the angle 

 separating the posterior and lateral surfaces, there is a small depression 



