270 MAMMALIAN ANATOMY 



VARIATIONS IN FORM AND DEVELOPMENT. 



In some specimens the superior surface of the prespheuoid is re- 

 duced to a very narrow median strip by the increased width of the 

 marginal frontal articular surfaces. 



The optic groove may be sharply defined from the olfactory surface 

 by a continuous ridge, which is either transverse or V-shaped. In one 

 specimen examined, the middle of the optic groove exhibited a pointed 

 triangular spine, which was directed backward and, by almost touching 

 the olivary eminence, formed an incomplete transverse canal. 



In the majority of sphenoids the anterior clinoid processes are well 

 developed ; occasionally they are prolonged backward as long spines. 

 Not infrequently the posterior edge of the orbitosphenoid, just lateral 

 to the anterior clinoid process, presents a second small process ; the 

 intermediate groove lodges the ophthalmic artery. 



The sides of the sella turcica rarely show middle clinoid processes. 

 When present they are very small and placed near the anterior end. 

 The diameters of the sella turcica are variable, even in sphenoids of 

 the same size ; its lateral boundaries are occasionally clearly defined 

 as far back as the lingula. 



In some specimens the large notch in the posterior border, medial 

 to the lingula, is bridged by bone and converted into a foramen. 



The sharp edge of the alisphenoid which overhangs the inner 

 openings of the foramina varies widely in form and development. It 

 is sometimes so deeply eniarginate, especially behind, that the elevated 

 part of the great wing is reduced to a narrow strip, and the lower flat 

 medial part is largely uncovered ; in a few specimens the extracranial 

 opening of the foramen ovale was visible when the bone was viewed 

 squarely from above. 



The inner surface of the parietal angle may be almost completely 

 occupied by a roughened surface for the frontal; in other cases this 

 articulation is confined to the free edge only. A true parietal articu- 

 lation appears to be always present, although in extent sometimes 

 greatly reduced. 



The small meniugeal foramen between the small and great wings 

 is always confined to the suture ; it is generally very small, but may 

 reach a considerable size. 



The anterior aspect of the anterior sphenoid in some specimens is 

 very much wider than high. The spheno-turbinal ridges on the outer 



