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 formula?. 



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' +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 + jj'i'*; 



23. 



si+p+p'i'; 



24. 



s+p'i'+p + i] 



25. 



spp'i + i'*; 



26. 



spp'i' + i*; 



27. 



spp + i -f i'. 







Those with a star appended 



are represented 



in oui 



• Museum. 



Another source of variation is the union of one element, the 

 preinterparietal, with the parietal bone. Of this I have only 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 



