IOWA AGADEMY OF SCIENCES. 119 



THE HOMOLOGY OF THE "INCA" BONE. 



BY C. C. NUTTING. 



About two years ago, while examining the interesting series of prehis- 

 toric skulls in the collection of the Davenport Academy of Sciences, the 

 •writer became involved in an attempt to account for the supernumerary 

 bone which some one has marked the "inca" bone. What the significance of 

 the name may be, I do not know, but the significance of the fact is the object 

 of the inquiry involved in this paper. 



In a series of about twenty skulls examined by me there were at least 

 six which exhibited the so-called "inca" bone, which is a portion of the 

 occipital, separated from the remainder by a very distinct suture extending 

 across the bone, following the "superior curved line," and about one-half 

 inch above it. This suture is quite constant in position in every skull show- 

 ing the "inca" bone. 



The portion of the occipital which is thus cut off shows a tendency to 

 itself divide into two or three pieces. But the sutures in this case are not 

 constant in position and may, in fact, occur in almost any portion of the 

 "inca" bone. 



In attempting to homologize this peculiar bone, three possibilities occur; 



First— The inca bone is the homologue of the supraoccipital of certain 

 of the lower mammalia. 



Second— The, inca bone may be simply an enormously developed wormian 

 bone. 



Third— It may be a persistent embryonic character. 



As to the first hypothesis, i. e., that it is the supra occipital, we find that 

 the supraoccipital in lower mammalia reaches to and forms part of the 

 borders of the foramen magnum. The "inca" bone, on the contrary, is 

 always remote from the foramen magnum, being above the superior curved 

 line. It can thus be seen that the bone in question cannot be the supra 

 occipital. 



The second hypothesis, i. e., that we have here merely an enormously 

 developed wormian bone, would, at first thought, seem to be unworthy of 

 serious consideration. But Gray' says, in his classic Anatomy: 



"They (the wormian bones) vai-y much in size, being in some cases not 

 larger than a pin's head, and confined to the outer table; in other cases so 

 large that one pair of these bones may form the whole of the occipital bone 

 above the superior curved lines." 



This is the extent of the ''inca" bone in all cases, and in at least one 



iGray's Anatomy, Eleventh Edition, p. 18i. 



