NO. 3 COMPARATIVE HISTOLOGY OF FEMUR — -FOOTE 91 



The medullary caual is full. Medullary index, 67%. 



Structure. — With the exception of a few small and i^oorly developed Ha- 

 versian systems of the (lb) differentiation in the posterior and inner walls, the 

 section is composed of lamellte with oval lacunae and straight canaliculi. The 

 specimen shows very little differentiation of structure. The bone is uniform. 



Type I. 



KIGHT FEMUE OF ECHIDNA. EGG-LAYING MAMMAL. NO. 1735.5, AMER. MUS. NAT. HIST. 



Pl. 9, Fig. U9. Stn. Tab. V 



Antero-posterior diameter of bone, 6 mm.; lateral, 11.5 mm. 



Antero-posterior diameter of medullary canal, 3.5 mm. ; lateral, 6.5 mm. 



The medullary canal is full. Medullary index, 49%. 



Structure. — A few primitive Haversian systems of the (lb) differentiation 

 are found in the inner and outer walls. The section, for the most part, is com- 

 posed of basic bone substance with round and oval lacuiiEe and bushy canaliculi, 

 crossed at various angles by branching canals. Around the medullary canal 

 the lacunae are longer and the lamellae denser than elsewhere. In the outer wall 

 of the medullary canal cancellous bone occurs. Tbe bone shows some departure 

 from the first type by its crude Haversian systems. 



Type I. 



LEFT FEMUR OF ORNITHORHYNCHUS. DUCK BILL. EGG-LAYING MAMMAL. 



NO. 13354 U. S. NAT. MUS. 



Pl. 9, Fig. 170. Syn. Tab. \ 



Antero-posterior diameter of the bone, 4 mm. ; lateral, 5 mm. 



Antero-posterior diameter of the medullary canal, 2.5 mm.; lateral, 3.5 mm. 



The medullary canal is full. Medullary index, 79%. 



Structure. — The section is divided about equally uito two concentric parts 

 or rings not separated by any well marked boundary. Tlie external half is com- 

 posed of basic bone substance with a great many oval and round lacunae and 

 short, bushy canaliculi. In some places, as in the outer and inner ridges, crude 

 Haversian systems appear. Wide canals arranged transversely, obliquely, and 

 concentrically occur at frequent intervals. The internal lialf consists of lamella' 

 with long lacunar and canaliculi, interrupted in the outer wall by Haversian 

 systems of the (lb) differentiation. The lamella are crossed by canals, and in 

 the anterior wall the canals run concentrically. 



Type I-III, lb. 



