2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 6l 



bone, together with a number of combinations of these types, is estab- 

 lished. They might be called the early, intermediate, and advanced, 

 or, more definitely, the undifferentiated, laminar, and Haversian-sys- 

 tem types. As a matter of convenience they will be referred to as 

 the first, the second, and the third types, respectively. They are illus- 

 trated in plate I. They are doubtless connected intimately with vascu- 

 lar development, and may be defined as follows : 



The first type (pi. i, fig. i) is composed of homogeneous bone 

 substance enclosing more or less numerous lacunae, from which 

 radiate their minute canaliculi. It is very poor in vascular canals. 

 The lacunas present a simple concentric arrangement; they may be 

 comparatively few or many in number ; they may be round or oval 

 in shape, with few or again many canaliculi ; and the bone may show 

 an approach to the simplest form of lamination. 1 



The second type (pi. i, fig. 2) is composed of groups of concentric 

 laminae which show vascular canals running parallel to tf\e axis of the 

 bone, as well as about the laminae, and are frequently crossed by 

 smaller canals running in various directions. The lacunae are con- 

 siderable in number, and may be oval, or long and narrow, according 

 to the species. 



The third type (pi. 1, fig. 3) is composed of Haversian systems, 

 such a system is defined by Cunningham as follows : " The Haversian 

 system consists of a central or Haversian canal which contains a vessel 

 of the bone. Around this osseous lamellae are arranged concentri- 

 cally, separated here and there by interspaces called lacunae, in which 

 the bone corpuscles are lodged. Passing from these lacunae are many 

 fine channels called canaliculi. These are disposed radially to the 

 Haversian canal and pass through the osseous lamellae. They are 

 occupied by the slender processes of the bone corpuscles." 



These three types, either singly or in combination, enter into the 

 formation of the femora of all animals ; and there is no suggestion of 

 any additional form of bone structure. Taken as a whole, combina- 

 tions of types are more common in the structure of bone than single 

 types, and are more frequent in the mammals than in the classes below 

 them. 





1 The term lamella is restricted in this paper to the small concentric layers o 

 bone surrounding the Haversian canal, while that of lamina; is applied to the 

 larger more or less irregular rings of bone that run concentrically in relation 

 to the medullary canal. 





