130 FORMATION OF SHELLS OF ANIMALS, ETC., 
freely, appear to enclose in their circular meshes the 
rounded, osseous particles of which the bone is composed. 
In the bone of the palate of the Myliobatis, the lacunz 
appear to be replaced by Haversian canals, from which 
very large canaliculi proceed, so that there being only one 
set of passages besides canaliculi, either lacune or Haver- 
sian canals must be wanting. The Haversian canals, and 
Haversian systems, as they are called, present great variety. 
Some of the most irregular and ill-defined of these canals 
are called Haversian spaces. As to the Haversian system, 
it may be observed that in some bones they are distinct, 
and separated by interstitical portions of osseous tissue ; 
but, in others, as in those of the bird, they are blended 
together. Besides, in the bones of fishes generally, and in 
some of those of the larger reptiles, as the crocodile, one 
part of a specimen will present Haversian systems, whilst 
in other parts they are altogether absent. Lastly, as a 
specimen of bone remarkable for the irregularity of its 
various component structures, [ may mention the cementum 
of ruminants, which often presents Haversian canals, 
lacunee, and canaliculi, all of the most irregular and ap- 
parently abnormal character. Indeed, thin sections of 
some of the densest of the sclerous vegetable tissues have, 
excepting the fact of the total absence in those of Haver- 
sian canals, as much the appearance of ordinary bone as 
this substance. And in good specimens of the sclerous 
tissue in progress of development, the lacunz and cana- 
lieu can be seen to be formed by the partial coalescence 
of previously existing sclerogen particles, in the same 
manner as was noticed of the osseous particles, when de- 
scribing the lacune and canaliculi of bone. ‘The inner- 
most portion of the shell of the unripe peach-nut is well 
adapted for the examination of this process, especially if 
a 
