142 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 33 



Structure. — The section is surrounded by a narrow horseshoe of lamellae, 

 widest in the inner wall. The central ring forms the greater portion of the 

 width of the wall. It reaches the surface of the posterior ridge and constitutes 

 nearly all of the posterior wall. The ring is composed of well developed Ha- 

 versian systems. The internal circumferential lamella? form a ring of lamellae 

 and laminae extending in various directions. 



Type II-III, C. 



The femora of ten dogs were examined and no two of them showed the 

 same structure. They ranged from a second to a second and third combination. 

 In the combinations the proportions of the units varied greatly. 



LEFT FEMUR OF LEPUS CUNICULUS. RABBIT. CR. MED. COLL. 



Pl. 19, Fig. 286. Syn. Tab. VII 



Antero-posterior diameter of bone, 5.5 mm. ; lateral, 7.5 mm. 



Antero-posterior diameter of medullary canal, 3.5 mm.; lateral, 5 mm. 



The medullary canal is full. Medullary index, 74%. 



Structure. — Around the bone is a ring of lamellae of varying thicknesses. As 

 a whole, it is narrow, and, in the posterior wall, merges into oblique laminae 

 which join the internal circumferential lamellae. The lacunae are long and nar- 

 row and the canaliculi are long and branching. 



There is a central ring of incomplete Haversian systems and short, irregu- 

 lar laminae occupying the anterior and inner wall. In the posterior wall this 

 ring is interrupted by oblique, well developed Haversian systems and laminae 

 extending from the internal to the external circumferential lamellae. In the 

 outer wall there are wide, oblique canals separating irregular laminae extend- 

 ing from the internal to the external lamelte and interdigitating with exten- 

 sions from the iieriosteum. These two oblique arrangements enclose a small 

 crescent of irregular systems and lamellae. The lacunae are oval or long and the 

 canaliculi are bushy. 



Internal circumferenlial lamella^ of varying thickness and well developed 

 surround the medullar}' canal. In tlie inner and posterior wall it merges into 

 oblique, wide laminae, separated by an oblique row of complete Haversian sys- 

 tems. To the outer side of this row of systems are three or four wide, oblique 

 laminae which appear to be extensions of the internal lamellae. 



Type I-II-III, C. 



RIOHT FEMUR OF PROCYON LOTOR. RACCOON 



]^L. 20, Fill. 287. Syn. Tais. VII 



Antero-posterior diameter of bone, 9 mm.; lateral, 10 mm. 

 Antero-posterior diameter of medullary canal, 6.5 mm.; lateral, 7 mm. 

 The medullary canal is full. Medullary index, 100%. 



