86 ILLINOIS BIOLOGICAL MONOGRAPHS [86 



The supraoccipital. This is a large unpaired bone which forms the median 

 cranial roof behind the frontals. Its anterior margin is divided into halves 

 by the posterior end of the posterior fontanelle, on the margins of which it meets 

 the frontals (Fig. 10). The portions of the bone along the fontanelle are raised 

 above the level of the surrounding bones and form a ridge which is continuous 

 anteriorly with a similar ridge on the frontals and posteriorly with the medial 

 surface of the occipital spine. Muscles of the opercular and mandibular 

 apparatus have their insertion along the sides of these ridges. At the base of the 

 ridge the supraoccipital interdigitates with both the sphenotic and squamoso- 

 pterotic. 



Near the posterior margin of the dorsal surface, a crest, continuous with that 

 of the epiotic, meets the longitudinal ridge at right angles and a cavity is formed 

 by their intersection (Fig. 10). The crest curves postero-dorsally and forms 

 the lateral margin of the dorsal surface of the spina occipitis, a triangular 

 osseous splint which projects posteriorly from the dorsal surface of the cranium. 

 Between the crests of the two sides the dorsal surface of the spine has the shape 

 of a trough, the floor of which is irregularly excavated for the insertion of muscle 

 fibres. The ventral edge of the spine is connected to the posterior dorsal 

 surface of the supraoccipital proper by a thin osseous sheet, which, as noted 

 above (Fig. 29), is developed by the ossification of the connective tissue 

 between the anterior muscles. 



Behind the crest the main portion of the supraoccipital bone descends 

 on the posterior surface of the cranium as far as the exoccipital bones (Fig. 9). 

 Laterally, there is an interdigitation between the upper medial margin of the 

 epiotic and the supraoccipital, which continues anteriorly into the crest. The 

 ventrolateral margin of the supraoccipital is separated from the epiotic by a 

 strip of cartilage, continuous ventrally with that between the epiotic and exoc- 

 cipital. The ventral margin of the supraoccipital interdigitates with the dorsal 

 margin of the exoccipital in the wall of the foramen magnum, where there is 

 also a strip of cartilage between them. The supraoccipital forms a very small 

 part of the wall of the foramen. Just above the foramen the supraoccipital is 

 embraced by the anterior ventral surface of the anterior spine of the compound 

 vertebra, which, as stated in the account of the development of this region, is 

 the neurapophysis of the third neural arch, the first two neural arches being 

 modified as parts of the Weberian apparatus. The whole external surface of 

 the posterior face of the bone is rugose and bears ridges for the attachment of 

 muscle fibres. 



The internal surface of the bone is very smooth and for the most part is 

 only a superficial lamella on the cartilage around the dorsal ends of the septa 

 semicircularia (Fig. 7). Anteriorly, on each side, it interdigitates with the 

 frontal and laterally with the sphenotic; each side is separated from the pos- 

 terior end of the sphenotic by a small area of cartilage at the dorsal end of the 

 septum semicircularis anterior. The bone extends over the cartilage between 



