148 



SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE 



vol. 35 



The central ring is composed of Haversian systems of the (lb) differentia- 

 tion. Their lacunae are generally oval or round, in a few places long and nar- 

 row, and are at some distance from the Haversian canals. Their canaliculi are 

 long. The rings break through the external circumferential laminae at the 

 inner ridge and form the whole width of the ridge, from the internal circum- 

 ferential lamellae to the external surface. The internal circumferential lamellae 

 of the different widths surround the medullary canal and in some places form 

 cancellous structure. Their lacunae are long, narrow, and well developed. 



Type II-III, la, lb. 



RIGHT FEMUR OE HAPLODONTIA OLYMPICA. SEWELLEL, MOUNTAIN BEAVER, OR FARMER 



Pl. 20, Fig. 297. Syn. Tab. VII 



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

 Antero-posterior diameter of medullary canal, 3 mm. ; lateral, 1.5 mm. 

 The medullary canal is full. Medullary index, 26%. 



Structure. — The section is composed of a confusing mixture of lamellae, 

 laminae, and Haversian systems of various differentiations. A narrow ring 

 of lamellae and crude Haversian systems surrounds the bone. The central ring 

 is composed of crude Haversian systems, following no definite plan of arrange- 

 ment and representing no definite state of development. The ring is crossed 

 obliquely by a few laminae and separated into two parts by concentric lamellae. 

 The internal circumferential lamellae are incomplete. 



Type I-II-III, la, lb. 



LEFT FEMUR OF ERETHIZON. PORCUPINE. CR. MED. COLL. 



Pl. 20, Fig. 298. Syn. Tab. VII 



Antero-posterior diameter of bone, 9.5 mm. ; lateral, 7.5 mm. 

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

 The medullary canal is full. Medullary index, 12%. 



Structure. — With the exception of the posterior wall the section is sur- 

 rounded by a narrow band of external circumferential lamellae. The posterior 

 wall is composed of crude Haversian systems and inter-Haversian bone sub- 

 stance with oval lacunae and bushy canaliculi. 



The central ring is composed of Haversian systems and inter-Haversian 

 bone substance with oval lacunae and bushy canaliculi. 



Internal circumferential lamellae, crossed by many radiating canals and 

 interrupted by Haversian systems of the (la) and (lb) differentiation, surround 

 the medullary canal. 



Type I-III, la, lb, C. 



