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SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE 



vol. 35 



tral division or ring are found numerous Haversian canals situated nearer to 

 the internal than to the external division. The canals run parallel with the' 

 medullary canal. They are surrounded by clear areas of bone substance which 

 are crossed radially by very fine canaliculi from adjacent lacunae. The canals 

 with their canaliculi and lacunae are crude outlines of Haversian systems of the 

 (la) differentiation. The bone has the threefold division. 

 Type I-III, la. 



X. OTHER MAMMALS NOT INCLUDING MAN 

 One hundred and thirty-three femora were examined. 



General Character of the Femur 



The general shape of the femur varies very considerably. Some bones are 

 triangular, many elliptical, some round, some indeterminate, and a few are 

 plano-convex. The majority of them are elliptical. 



The medullary canals, with one or two exceptions, are full of marrow, and 

 a large number have cancellous bone, the meshes of which are filled with mar- 

 row. The medullary surfaces are generally rough or irregularly corrugated. 

 The medullary index varies from 9% to 289% with an average of 63.3%. 



The type of structure varies greatly. Nearly all types and type combina- 

 tions in their various stages of differentiation are found. The bone units of 

 monotremes and marsupials present an earlier differentiation than the higher 

 mammals. 



The first type bone is present in many genera and may be associated with 

 the undeveloped third type of the (la) differentiation. A number of mammalian 

 femora present a well marked second type structure. The laminae are much 

 better developed than in birds. In most of these bones Haversian systems are 

 found to the greatest extent in the posterior ridges. The pure third type of the 

 (C) differentiation occurs only in a few mammals. Of these the African 

 elephant is the best example. The majority of mammalian femora exhibit type 

 combinations. These may be first and third, second and third, or first, second, 

 and third in some form of differentiation. The majority are composed of the 

 first and third types. 



Detailed Examination 



left femur of tupaia. tree-shrew. amer. mus. nat. hist. 



Pl. 9, Fig. 108. Syn. Tab. V 



Antero-posterior diameter of bone, 3 mm. ; lateral, 2.5 mm. 

 Antero-posterior diameter of medullary canal, 2 mm.; lateral, 1.5 mm. 



