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SMITHSONIAN CONTKIBUTIONS TO KNOWLEDGE 



VOL. 35 



are found. None of these, with the possible exception of the first, have reached 

 their full development. The second is found in a very incomplete and an 

 advanced stage and forms the structure of a large number of femora; while 

 the third has assumed a more complex form than that found in reptiles. The 

 Haversian systems are comparatively large, the Haversian canals are small 

 and around them are concentrically arranged round or oval lacunae with intri- 

 cate networks of canaliculi. This is the (Ic) stage of Haversian differentiation 

 and is characteristic of birds. 



Detailed Examination 



bight femur of cyanocitta stelleri azteca. aztec jay. no. 2874, 



amer. mus. nat. hist. 



Pl. 5, Fig. 74. Syn. Tab. Ill 



Antero-posterior diameter of bone, 3 mm. ; lateral, 2.5 mm. 

 Antero-posterior diameter of medullary canal, 2 mm. ; lateral, 1.5 mm. 

 The medullary canal is full. Medullary index, 68%. 



Structure. — The section is composed of bone substance with oval lacunae 

 and bushy canaliculi surrounding the medullary canal. The bone substance is 

 partially separated into laminae by short canals. The bone shows but little 

 differentiation of structure. 



Type I-II. 



LEFT FEMUR OF MERGUS SERRATOR. RED-BREASTED MERGANSER. NO. 3117, 



AMER. MUS. NAT. HIST. 



Pl. 5, Fig. 75. Syn. Tab. Ill 



Antero-posterior diameter of bone, 6.5 mm. ; lateral, 4.5 mm. 

 Antero-posterior diameter of medullary canal, 4.5 mm. ; lateral, 2.5 mm. 

 The medullary canal is full. Medullary index, 68%. 



Structure. — The section is composed of bone substance with oval lacunae 

 and bushy canaliculi, separated into incomplete laminae by short, concentric, 

 branching canals. A few vascular canals running longitudinally appear in the 

 posterior inner and anterior wall. In the bone substance are a few Haversian 

 systems of the (Ic) stage of development, and a single better differentiated 

 system occurs in the tip of the posterior ridge. The lacunae are oval. A nar- 

 row ring of internal circumferential lamellae with long lacunae and straight 

 canaliculi surrounds the medullary canal. 



Type I-II-III, Ic. 



