NO. 3 COMPARATIVE HISTOLOGY OF FEMUR FOOTE 147 



RIGHT FEMUR OF DIDELPHYS VIRGINIAN-A. OPOSSUM. CR. MED. COLL. 



Pl. 20, Fig. 205. Syn. Tab. VII 



Antero-posterior diameter of bone, 7 mm. ; lateral, 8.5 mm. 



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



The medullary canal is full. Medullary index, 42%. 



Structure. — The bone presents a rudimentary appearance. It is composed 

 of two wide external lamellar bands of incomplete formation, separated by a 

 very narrow band of imperfectly developed Haversian systems, the whole oc- 

 cupying two-thirds of the posterior, outer, and anterior wall. The lamellar 

 bands are composed of bone substance with large, oval lacuiiEe and extensive, 

 bushy canaliculi forming an intricate network. At short intervals radiating 

 canals appear, giving a bush-like appearance to the band. Just internal to this 

 lamellar band is a narrow crescent of very incomplete Haversian systems occu- 

 pying the anterior, outer, and posterior wall. The systems are of the (lb) 

 differentiation. Around the medullary canal of the anterior, outer, and pos- 

 terior wall, internal circumferential lamellfe are well developed, reaching their 

 greatest thickness in the outer wall. Their lacunse are long and narrow and 

 their canaliculi are long, straight, and branching. 



The inner wall of the bone is extended in the form of a heavy ridge. It 

 is composed of bone substance with heavy, oblique canals, from which are sent 

 off dense networks of large canaliculi. This peculiar arrangement forms the 

 external half of the ridge. The internal half is composed of incomplete Ha- 

 versian systems, arranged in oblique rows, converging to a central point in the 

 middle of the ridge. No internal circumferential lamellse are found in this 

 region. 



Type I-III, lb. 



left femur of manis. ant-eater. no. 8351, u. s. nat. mus. 

 Pl. 20, Fig. 296. Syn. Tab. VIT 



Antero-posterior diameter of bone, 9 mm.; lateral, 12.5 mm. 



Antero-posterior diameter of medullary canal, 4 mm.; lateral, 5.5 mm. 



The medullary canal is full. Medullary index, 24%. 



Structure. — The three structural divisions appear in a somewhat modified 

 form. A thick, incomplete ring of crude circumferential laminas surrounds the 

 section, excepting the posterior ridge. The laminae are wide and composed of 

 lamellae having round and oval lacunae with bushy canaliculi. They are separ- 

 ated by irregularly shaped canals which appear fragmentary in the section. 

 In some situations the canals are branching and arranged in plexus form. The 

 laminae are frequently interrupted by Haversian canals of the (la) differen- 

 tiation. 



