of the Occipital bone. 
151 
The four normal elements, when distinct, are arranged two 
laterally and two medially and superior. The former are named 
interparietals, and the latter preinterparietals. The union of inter- 
parietal and preinterparietal on each side takes place in general 
with the fourth month, and the combined lateral plates begin to 
unite medially and below by the end of that month. At birth, 
as is well known, the entire hinder part of the occipital is usually 
a single piece shewing marginal traces of the suture below supra- 
occipital and interparietals as well as medially and above, 
between the contiguous preinterparietals. Almost all possible 
deviations from normal union of these elements may occur, and 
these may be tabulated by a simple series of formulae. 
Taking the type form in which all the elements coalesce, 
supraoccipital, interparietals and preinterparietals, and distinguish- 
ing the right from the left of these paired elements by affixing 
a dash to the initial of the latter, it may be represented by the 
formula sii'pp'. On the other hand, the cases in which each 
element remains discrete so that the single occipital bone is 
represented by an archipelago, the formula would be 
s + i + i' +p+p'. 
Between these extremes there are about 28 possible intermediate 
combinations, of which the most interesting are the forms known 
as the interparietal bone which is s+pp'ii', and that called 
epactal bone which is sii' +pp. The other possible combinations 
are 
5. 
s + pp +i + i'*; 
6. 
si + pp + i*; 
7. 
si' +pp + i*. 
8. 
si' + pp'i*; 
9. 
s + pp'i + i'*; 
10. 
si -{-pp'i'; 
11. 
s + i + pp'i*; 
12. 
sp'ii' + p\ 
13. 
sii' + p + p ; 
14. 
si ' + p + p' -\-i*; 
15. 
si+p-\-p'+i'; 
16. 
spii' -v p * ; 
17. 
sp + p + i + i'*\ 
18. 
sp'i' + pi; 
19. 
s + pi + p'i'; 
20. 
si' + p + pi ; 
21. 
s + pi + p'.+i'; 
22. 
spi + p'i'*; 
23. 
si + p + p'i'; 
24. 
s +p'i'+p+i; 
25. 
spp'i + i'*; 
26. 
spp'i' -f i*; 
27. 
spp' + i 4- i'. 
Those with a star appended are represented in our Museum. 
Another source of variation is the union of one element, the 
preinterparietal, with the parietal bone. Of this I have ouly one 
specimen (spii' +p' par). 
Over and above the varieties of the combination of the normal 
centres of ossification there is a second and more prolific series 
of varieties due to the occurrence of new centres along the line 
of the lambdoidal suture. These are numerous and perplexing 
