XO. 3 COMPARATIVE HISTOLOGY OF FEMUR FOOTE 109 



concentric course; while in the internal they form two wide crescent-shaped 

 bands around the medullary canal. In the mid-line of the anterior wall the 

 systems are arranged in a narrow column from one surface to the other. 



The posterior ridge is composed of Haversian systems, large, small, and 

 irregular in shape. Internal circumferential lamellas in a narrow ring enclose 

 tlie medullary canal. 



Type II-III, C. 



RIGHT FEMUR OF ELEPHAS AFRICANUS. AFRICAN ELEPHANT. NO. 35185, 



AMER. MUS. NAT. HIST. 



Pl. 13, Fig. 216. Stn. Tab. VI 



Antero-posterior diameter of bone, 108 mm. ; lateral, 83 mm. 



Antero-iDosterior diameter of medullary canal, 65 mm. ; lateral, 49 mm. 



The medullary canal is full. Medullary index, 55%. 



Structure. — With the exception of a very narrow fragmentary ring of ex- 

 ternal circumferential lamellaj with long lacuna and straight canaliculi, the sec- 

 tion is composed almost entirely of Haversian systems of the (C) differentia- 

 tion. Inter-Haversian lamellse are present in some portions of the bone. The 

 systems form the external boundary of the inner anterior wall where the lamella; 

 are deficient. Numerous cross canals unite the systems. The lacunar of the 

 whole section are long and narrow and their canaliculi are long, straight, and 

 thickly set. The internal circumferential lamellae are fragTuentary. 



Type III, C. 



FEMUR OF CHOLCEPUS DIDACTYLUS. TWO-TOED SLOTH. NO. 104593, U. S. NAT. MUS. 



Pl. 12, Fig. 217. Stn. Tab. VI 



Antero-posterior diameter of bone, 16 mm. ; lateral, 11 mm. 



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



The medullary canal is cancellous. Medullary index, 9%. 



Structure. — The section has a long posterior ridge and exhibits the three 

 structural divisions. The external circumferential lamelte form a narrow ring 

 around the bone excepting at the posterior ridge. The lacunas are long with 

 straight canaliculi in some places and oval with bushy canaliculi in others. 



The central ring is composed of very distinct Haversian systems with little 

 inter-Haversian lamella;. They are of a high structural type. Their lacuna; 

 are long and their canaliculi are long and straight. Some of the Haversian 

 systems are united by cross canals, but not many. The systems are well 

 developed but many, especially around the medullary canal, show senile changes. 



The internal circumferential lamellfe take the form of a thick ring of can- 

 cellous bone. The lacuuse are well developed. 



Type III, C, senile. 



