6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 6l 



The first type is present in certain genera. It was found in 50 

 different genera of bats and a number of genera of shrews. In a few 

 of these forms the division of the bone substance into three concentric 

 rings has occurred. In one genus, the Pteropus, this modification is 

 well marked, and beyond that, rather crude Haversian systems are 

 found in the central ring, the bone showing thus an advance toward 

 the third type. 



Quite a large number of mammalian femora present a well marked 

 second type structure. The laminae are well developed, and enclose 

 long, narrow lacunae, with straight canaliculi. In all these bones 

 Haversian systems are found in the posterior ridge corresponding .to 

 the linea aspera. A few mammals show a pure third type of bone 

 structure. In such animals the first and second types are eliminated 

 and fully developed Haversian systems have taken their places. But 

 only three or four of the 178 mammalian femora (other than bats and 

 shrews) examined were composed of .this type. 



By far the greatest number of mammalian femora shows combina- 

 tions of the first and third type. In these bones the structural units 

 are well developed, but vary much in proportions. But they fre- 

 quently occupy the same relative positions. The laminae, with bone 

 structure of the first type, are external and internal, while the Haver- 

 sian systems occupy the central ring. The second and third types 

 form the structure of also a large number of mammals. In this com- 

 bination the units are well developed and about equally important. 

 Finally, in still another large group of mammals, the femora show all 

 the first, second and third types of bone structure, in varying pro- 

 portions. 



Looking over the mammals as a whole, it is noticed that their 

 femora exhibit structural differentiation much more advanced and 

 definite than that observed in the femora of other animals. From 

 species to species there are many variations. 



It is a peculiar fact that amphibians, reptiles, birds, and mammals 

 all present, though in a widely varying proportion, the first type as 

 well as some form of advanced type of bone structure. The advanced 

 type is the variable factor and occurs in the greatest variety of forms 

 and combinations. The early or first type differs merely in simplicity. 

 It is more simple in the amphibians than in the mammals. The third 

 type, on the other hand, in amphibians, merely a suggestion, is better 

 developed in reptiles, still better in birds and in mammals reaches its 

 highest state of advancement. Clean cut, well developed third type 



