NO. 3 COMPARATIVE HISTOLOGY OF FEMUK FOOTE 59 



Structure. — The seetion is composed of lamella? with oval laciime and bushy 

 canaliculi, incompletoly separated into lamiiisr which surround the medullary 

 canal. In the posterior wall the crude lamina? are frequently interrupted hy 

 Haversian canals of the (lb) differentiation. In the internal lamina? of the 

 anterior wall are several crude undeveloped and a few fairly well developed 

 Haversian systems. The medullary canal is occupied by cancellous bone. 



Tvpc I IT 111. Th. 



RIGHT FEMUK OF TESTUDO (gOPHERUS) POLM'HEMUS. NO. 7555, U. S. NAP. .MUS. 



PL. 4, Fig. 73. SyxV. Tab. II 



Antei-o-posterior diameter of bone, 8.5 mm. ; lateral, 6 mm. 



Antero-posterior diameter of medullary canal — cancellous bone. 



The medullary canal is full. Medullary index, 0. 



Structure. — The section is composed of incompletely formed, concentric 

 laminfe which constitute the anterior, outer, and posterior wall. The laminae 

 are crossed by short, radiating canals and interrupted by Haversian systems 

 of the (la) differentiation. Beneath the laminae of the anterior and outer 

 wall are Haversian systems of the (lb) stage of development and the posterior 

 ridge is made ui) ahnost entirely of Haversian systems of the same develop- 

 ment. The inner wall is composed of lamellae. 



Type 1 III, Ta, 11.. 



VIII. BIRDS 



Forty femora were examined. 



General Character of the Femur 



The femora of birds vary considerably in shape. A few are triangulai-, 

 many are elliptical, and some are circular. The majority of them are elliptical 

 and tlieir antero-posterior diameters are longest. The meduUaiy contents pre- 

 sent a variable character. In some femora the medullary canals are full of 

 marrow; in some, of cancellous bone, the meshes of which are filled with mar- 

 row; while in others, the canals are empty or occupied by trabeculae only. About 

 half of the femora examined have no contents. 



The medullary surfaces also vary somewhat in character. In those (ianals 

 filled with marrow and blood vessels the surface is uneven, while in those 

 which have no contents the surface is smooth. In these bones the walls are thin, 

 the canals large, and the trabeculae are numerous. Tlie medullary index varies 

 from to 327%, with an average of 159%. 



The bone structures show considerable variation. The three single types 

 an.l many combinations of types, in an incomplete or complete differentiation. 



