NO. 3 



COMPARATIVE HISTOLOGY OF FEMUR FOOTE 



91 



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



Structure. — With the exception of a few small and poorly developed Ha- 

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

 section is composed of lamella? with oval lacunas and straight canaliculi. The 

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



Type I. 



RIGHT FEMUR OF ECHIDNA. EGG -LAYING MAMMAL. NO. 17355, AMER. MUS. NAT. HIST. 



Pl. 9, Fig. 169. Syn. 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 lacunas and bushy canaliculi, 

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

 the lacunas are longer and the lamelhe denser than elsewhere. In the outer wall 

 of the medullary canal cancellous bone occurs. The 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, Pig. 170. Syn. Tab. V 



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 into two concentric parts 

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

 posed of basic bone substance with a great many oval and round lacunas 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 half consists of lamella? 

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

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

 the anterior wall the canals run concentrically. 



Type I-III, lb. 



