124 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 35 



and the canaliculi are straight. An incomplete rina- of internal circumferential 

 lamellae surrounds the medullary canal. 

 Type I-III, la, C. 



LEFT FEMtTK OF PEDETES. JUMPING HARE. U. S. NAT. MUS. 



Pl. 15, Fig. 348. Syn. Tab. VI 



Antero-posterior diameter of bone, 10.9 mm. ; lateral, 9 mm. 



Antero-posterior diameter of medullary canal, 7.5 mm.; lateral, 6 mm. 



The medullary canal is full. Medullary index, 85%. 



Structure.- — The bone is surrounded by a lamellar ring of varying widths. 

 The lamellae are frequently interrupted by small, crude Haversian systems 

 of the (la) differentiation and crossed by a few canals. The lacunae are oval 

 and narrow and the canaliculi are bushy and straight. Underneath the lamellae 

 is an irregular ring of Haversian systems separated by lamellae and canals. 

 The systems are fairly well developed. In tlie inner wall the systems are inter- 

 rupted by a few laminae. The internal circumferential lamellae form a ring of 

 varying widths around the medullary canal. Tlieir lacimse are narrow and the 

 canaliculi are straight. 



Type I-III, la, C. 



RIGHT FEMUR OF BRADYPUS TRIDACTVLUS. THREE-TOED SLOTH. NO. 16871, 



AMER. MUS. NAT. HIST. 



Pl. 15, Fig. 249. Syn. Tab. VI 



Antero-posterior diameter of bone, 8.5 mm.; lateral, 13 mm. 



Antero-posterior diameter of medullary canal, 3 mm.; lateral, 4 mm. 



The medullary canal is full. Medullary index, 12%. 



Structure. — The section has three divisions. The external circumferential 

 lamellae form a wide rim around the bone. It is crossed radially by numerous 

 canals and interrupted frequently by normal and senile Haversian systems and 

 by Haversian canals of the (la) differentiation. The band of lamelte consti- 

 tutes half of the wall. The lacunae are oval and long and the canaliculi are 

 straight. 



The central ring is composed of complete and senile Haversian systems. 

 Various stages and degrees of senility are found. Some systems are entirely 

 gone; some occur in narrow rings; some show precipitation of inorganic ma- 

 terial around the Haversian canal ; and some show the whole systems involved 

 but still in position. Their lacunfe are long and oval. The internal circum- 

 ferential lamellae assume a cancellous form around the medullary canal. Tlieir 

 lacunae are long and the canaliculi are straight. 



