20 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 35 



MAMMALS 



The femora of mammals, not, including man, present the first, second, and 

 third type units and most of their combinations. As far as can be determined 

 microscopically, these units are the same as were present in the amphibians, 

 reptiles, and birds ; but on the whole, they show a more advanced differentiation. 



Since the complete Haversian system does not appear in amphibians nor in 

 reptiles nor clearly in birds, and does appear in mammals as a predominating 

 structure and often in a high degree of differentiation, its general presence to 

 such an extent and to such a degree of advancement points to the age of mam- 

 mals as the most important period in the history of bone development. 



For convenience in description the bats are treated separately since they 

 do not resemble other mammals as closely as they do some of the lizards. 



Bats. — Practically the whole order has been covered, and from the draw- 

 ings, descriptions, and tables it will be seen that the bat femora are generally 

 true to the first type bone. The sections are composed of lamellae with round, 

 oval, or long and narrow lacuna?, arranged concentrically around the medullary 

 canal. 



The sections are uniform in structure with the exception of a small number 

 in which we find the twofold or threefold lamellar division. Very few structural 

 variations have occurred in the whole order. In the genus Pteropus, which 

 included the largest bats, Haversian canals of the early differentiation are 

 present, and, generally speaking, the femora of the larger species have more 

 of these canals than those of the smaller (jA. 8, figs. 113-166). 



Other mammals. — The femora of the fetal sheep, calf, and pig of half-time 

 development and also those of many adult mammals were examined. 



In the fetal sheep the type of bone was an incomplete second with short, 

 wide, irregular communicating canals. In the femur of the adult sheep the type 

 was also second; but the lamina had become regular and concentric and the 

 canals between them much narrower and more regular. In the fetal calf the 

 type of bone was very indistinctly second, with numerous bizarre-shaped canals. 

 In the adult ox the type had become a well developed second and third. 



In the fetal pig the type was second and, with the addition of a few 

 Haversian systems in the posterior wall, remained second in the adult (pi. 11, 

 figs. 199, 200, 201). 



The most pronounced developmental change was observed in the femur 

 of the calf. 



The whole range of ditferentiation in minute bone structure reaches, gen- 

 erally speaking, the greatest advance in the mammals exclusive of bats. 



The often more or less vague character of the structure in lower animals 

 clears up to a marked degree, and the bone units stand out as finished products. 

 Types and type combinations are now distinct. Pure types are found in many 



