194 PROCEEDINGS OF THE ANATOMICAL AND ANTHROPOLOGICAL 



the influence of heredity on the frequency of metopisni, is that it is 

 extremely common in certain isolated districts, such as some islands 

 off the coast of Italy, in which it appears to be much more common 

 than it is on the mainland. 



Passing now to the occipital bone, we occasionally find a suture, 

 which runs transversely across the bone, from one lateral angle to 

 the other. This suture marks off a bone which occupies the angle 

 between the two parietal bones. This bone is called the interparietal 

 bone, or " Os Incae " ; the latter name being given to it on account of 

 the frequency with which it was found in the skulls of the Incas of 

 Peru. 



Now in skulls in which an in- 

 terparietal bone is present, we find 

 in accordance with the law pre- 

 viously mentioned, that there is an 

 increase in the length of the occipital 

 bone, or in other words in a direc- 

 tion at right angles to the line of 

 the suture. To determine the 

 amount of this increase, I measured 

 the longitudinal arc from the pos 

 terior margin of the foramen mag- 

 num, or " opisthion," to the upper 

 angle of the occipital bone, or 

 lambda, Fig. 3, and compared this with the longitudinal arc from the 

 opisthion to the middle of the nasal notch of the frontal bone, or 

 " nasion ". The average length of these arcs in the skulls with an 

 interparietal bone as compared with the average length of the same 

 arcs measured in an equal number of normal skulls of corresponding 

 nationality is shown in the following table, in which I have also shown 

 the average; capacity of these two groups of skulls, and the difference 

 between the average arcs and capacities. 



Fig. 3. Median longitudinal section of a 

 skull, showing the position of the lambda 

 (I..) in a normal skull, and its average 

 situation (!.'.) in skulls with .m inter- 

 parietal bone. F., frontal bone ; P., 

 parietal bone ; O., occipital bone ; N., 

 nasion ; Br., bregma ; Op., opisthion. 



