58 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 35 



Structure. — The section is surroiiiided by a narrow riii,^- (if laiiiellir witli 

 o\al lacunae and bushy canaliculi, interrupted by' a few Haversian canals of 

 the (la) differentiation. From tlie under surface of this ring is derived a 

 cancellous bone which entirely fills the medullary canal. The walls of the 

 meshes are composed of lamella' with oval lacuna; and bushy canaliculi. In the 

 posterior wall are a few Haversian systems of the (lb) stage of development. 



Type I-III, la, lb. 



LEFT FEMUR OK CHKVSEMYS I'K^I'A. PAINTED TURTLE. AMER. MUS. NAT. HIST. 



Pl. 4, Fig. iO. Syn. Tau. U 



Antero-posterior diameter of bone, 2.5 mm. ; lateral, 4 mm. 



Antero-posterior diameter of medullary canal — can<:;cllous bone. 



The medullary canal is full. Medullary index, 0. 



Structure. — The section is sun-ounded by a ring of lamelhc wiiii (i\;il 

 lacunar and bushy canaliculi, iiilci ruptcd by a few Haversian canals of the 

 (la) differentiation. Haversian systems of the (lb) stage of devehtimicnt arc 

 found in the posterior wall. From the lamellar ring is dcri\cd the cancelions 

 bone which occupies the whole medullaiy caiuil. 



Type II 11, la, lb. 



EIGHT FEMUR OF AROMOCHELYS ODOKATUS. MUSK TURTLE. AMER. MUS. NAT. HIST. 



Pl. a, Fig. 71. Syx. Tab. It 



Antero-posterior diameter of bone, 1.8 mm.; lateral, 1.4 mm. 



Antero-posterior diameter of mofhillary canal, 0.3 mm.; latei'al, 0.3 nnii. 



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



Structure. — The section is surrounded by a band of lamellae of various 

 widths. The lacuna? are long and their canalirnli aic long and straighl. I'uder- 

 neath this is a central ring of lamelhc with oval huMuuc and bushy caiialiciili. 

 In this ring are several Haversian systems of the (lb) dilferentiation forming 

 a circular row around the medullary canal. 



Around the medullary canal is a riiig of internal circumferential lamelhc 

 with long lacuna; and straight canaliculi. The bone shows the outlines of the 

 three dixisions — external and internal circuiriferential lamellae and the central 

 liim of lamellaj with Haversian systems. 



Type l-1 1 1, 111. 



RIGHT FEMUR OF PSEUDEMYS FLORIDANA. NO. 28417, U. S. NAT. MUS. 



Pl. I, Fig. 72. Syv. Tah. IF 



Antero-postei'ior diameter of bone, 4 mtn.; latei'al, .'! mm. 

 Antero-posterior diameter of medullary canal — cancellous bone. 

 Medullary index, 0. 



