182 MAMMALIAN ANATOMY 



VARIATIONS IN DEVELOPMENT. 



The external surface of the parietal may be almost flat or it may 

 be strongly convex ; the maximum convexity, or parietal eminence, 

 varying also in position. 



In some specimens the articular surface for the squamous of the 

 temporal is reduced to a narrow strip along the inferior border ; in 

 others it may occupy the lower fourth of the surface, rising still higher 

 at the anterior end. 



A curved ridge passing from the superior border at the anterior 

 end of its articulation Avith the interparietal to the upper third of the 

 anterior border often marks an attachment of the temporal muscle and 

 cuts off a superior triangle which is smooth and more dense than the 

 rest of the surface. 



One specimen in my collection exhibits behind the middle of the 

 bone a deep vertical groove which divides the external surface into 

 two parts. Whether this is the marking of a blood-vessel, the result 

 of an injury, or a defect in development is not possible to determine, 

 but it is interesting because a similar variation has been observed in 

 the human parietal. 



In some specimens a foramen may be seen near the middle of the 

 posterior border, and in one immature parietal there is a small opening 

 just above the articular surface for the squamous. 



The only variations observed on the internal surface are variations 

 in the groove and the foramina which are situated near the posterior 

 border and lodge or transmit the lateral and superior petrosal sinuses. 

 In some specimens there is a distinct groove for the lateral sinus 

 leading to a foramen above the middle of the border ; in others the 

 groove is present but the sinus enters the interparietal or the occipital, 

 and in others again there is neither groove nor foramen. The superior 

 petrosal sinus very often enters a foramen near the lower part of the 

 groove for the lateral sinus. 



THE INTERPARIETAL BONE. 



General Description. The Interparietal is the smallest of all 

 the cranial bones. It is single and unpaired, and situated at the back 

 part of the roof of the cranium, in the median line between the 

 diverging upper posterior corners of the two parietals and in front 

 of the upper end of the vertical plate of the occipital (Fig. 111)). 



