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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



fossae in this genus was of the nature of a groove. In location they 

 were in general more or less marginal and not infrequently rather 

 low. In the two femora of one adult, each with a pronounced oblique 

 fossa, the latter, after hollowing markedly the lateral border, extends 

 on the right bone largely, on the left partly, onto the anterior surface 

 of the shaft. 



The age differences in the incidence and development of the fossa 

 in the chimpanzees are very noticeable. There is a steady and marked 

 decrease from childhood to adult life in the " absent " and " trace " 

 records, which indicates that the fossa continues to originate and 

 develop during the growth period. Its greatest frequency as well as 

 its optimum development are evidently not reached in this genus 

 until within the adult jDeriod. The fossa occurs with strikingly less 

 frequency in the children than in the adults. 



SUMMARY OF OBSERVATIONS ON ANTHROPOID APES 



The gibbons and siamangs, so far as the hypotrochanteric fossa is 

 concerned, show conditions more like those of the lower apes than 

 those of the larger anthropoids. A distinct form is rare, occurring in 

 roughly 5 percent of the femora. 



In the three large anthropoid apes the fossa is frequent — as a 

 distinct to pronounced depression — in the adolescent and adult 

 femora, being present in 73.8 percent of the gorillas, 75.8 percent of 

 the chimpanzees, and 80.4 percent of the orangs. 



In most gorillas, and not infrequently in the chimpanzees, the fossa 

 is situated low on the shaft — decidedly lower than in humans ; in 

 the orangs the level of the hollow is nearly the same as that in man. 



In all the orangs and generally in the gorillas and chimpanzees the 

 fossa is partly to wholly marginal, involving the lateral border of 

 the shaft. Occasionally, the fossa will encroach on the anterior sur- 

 face, and in four orang femora it was completely displaced to this 

 surface. 



In one gibbon and four gorilla femora, instead of a circumscribed 

 fossa, there was a marked groove without any lower (distal) 

 boundary. 



In many of the gorillas, less commonly in the chimpanzees, and 

 occasionally in the orang, the fossa is spacious and rough and has 

 plainly served for the attachment of a powerful muscle, doubtless 

 the gluteus maximus. 



