14 SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE VOL. 35 



little in advance of the lamellar tj^pe; in others the advancement is more pro- 

 nounced; while in still others the type appears to liave 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. Laminae show incomplete and complete stages 

 of differentiation. 



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

 sepai'ated into indistinct lamime 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, iu the elongated meshes of which laminai are more clearly seen. The 

 lacunae are oval and the canaliculi are very numerous and reticular iu arrange- 

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



Complete: In still otlier eases the vascular canals form more or less com- 

 plete circuits, and the laminte are well defined iu clearly differentiated sheaths, 

 with completely develojied lamellae and long lacunae with straight canaliculi, and 

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

 fig. J; pLB, figs. 2, 5). 



The laminae have more individuality than the lamellae, and in a dried femur 

 of the second tyjie 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 laniinje were first observed in two amphib- 

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

 (pi. 2, fig. 14; 1)1. 3, tig. 36). In both of these bones they were only fairly well 

 developed and occupied irregiilar 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 laminae 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 laminae 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 lacunae which are present in birds, and the complete concentric 

 canals iind liigher grade lacunar differentiations which are present iu 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- 



