14 



SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE 



VOL. 35 



little in advance of the lamellar type ; in others the advancement is more pro- 

 nounced; while in still others the type appears to have reached its limits of 

 development (pi. A, figs. H, I, J; pi. B, figs. 2, 5). 



The exact mode of development of the lamina, as in the case of the 

 lamella, has not yet been traced. Lamina? show incomplete and complete stages 

 of differentiation. 



Incomplete: In some cases, as in birds, basic bone substance is partially 

 separated into indistinct laminae by a few, short, vascular canals having general 

 concentric positions. The lacuna? are round or oval and the canaliculi are short 

 and bushy and rather infrequent (pi. A, fig. H). In other cases the concentric 

 canals are lengthened and arranged in the form of a more or less complete 

 plexus, in the elongated meshes of which lamina? are more clearly seen. The 

 lacuna? are oval and the canaliculi are very numerous and reticular in arrange- 

 ment (pi. A, figs. I or 2). 



Complete: In still other cases the vascular canals form more or less com- 

 plete circuits, and the lamina? are well defined in clearly differentiated sheaths, 

 with completely developed lamella? and long lacuna? with straight canaliculi, and 

 are pierced quite regularly by vascular canals extending transversely (pi. A, 

 fig. J; pi. B, figs. 2,5). 



The lamina? have more individuality than the lamella?, and in a dried femur 

 of the second type they can be scaled off one after another. They are fairly 

 uniform in thickness, but in the incomplete stage vary considerably in the 

 length of their segments. The lamina? were first observed in two amphib- 

 ians, occurring singly in the femora of the Bufo americana and Hyla gratiosa 

 (pi. 2, fig. 14; pi. 3, fig. 36). In both of these bones they were only fairly well 

 developed and occupied irregular positions in reference to the medullary and 

 external surfaces. In the alligator and some turtles they alternated with con- 

 centric rings of crude Haversian systems. But it is in birds that lamina? first 

 become prominent as units of bone structure. In these animals the structure 

 of the femora examined presented stages of differentiation varying from a 

 very incomplete to an advanced, but not complete, character. It is not until 

 the lamina? reach the class of mammals that they show their highest develop- 

 ment, so that in birds they appear to occupy a transitional position. In fact 

 it is not difficult to distinguish between these units of the bird and the mammal 

 by the incomplete character of the concentric canals and the early differentia- 

 tions of the lacuna? which are present in birds, and the complete concentric 

 canals and higher grade lacunar differentiations which are present in mammals. 



THE THIRD TYPE 



This type is composed of Haversian systems as the main units of structure. 

 These systems form the whole bone or a larger proportion of the same. Nar- 



