Genera of the ^sTorth American Palaeozoic Bryozoa. 423 



" This examination can be made more easily than one might at 

 first imagine ; for neither the observation of the same individual, 

 at different stages of its development, nor the collection of a 

 series of specimens so as to represent all the phases through 

 which these little creatures pass successively, is required. 

 Indeed, since these polyps spring from each other, and do not 

 separate from their parents, each polypidom must present a long- 

 series of generations enchained to each other, and in each of 

 these series the relative age of living individuals must be indi- 

 cated by the place they occupy. To resolve the question which 

 we have put, it is sufficient, therefore, to study comparatively the 

 cells situated near the base of the polypidom, in its middle, in its 

 young branches, and toward the extremity of the latter ; for we 

 are certain that it is not only in this last place that living polyps 

 are found, as some authors affirm, but that they exist over almost 

 the entire extent of the polypidom. 



" After examining in this manner, with a sufficient magnifying 

 power, the cells of the Eschar a cermcornis^ I am quite convinced 

 that the mode of development of these stony cells is not that 

 which is usually admitted. 



^' Indeed, I have seen that not only does the general conforma- 

 tion of the cells change with age, but also that these changes 

 operate in a great measure on the exterior surface — that is to 

 say, on that side of their parietes, which, in the hypothesis of 

 their formation by layers, must exist from the first, and once 

 consolidated, ought to change no more, unless from exterior and 

 accidental frictions. 



" In the young cells whose partitions, although thin, have 

 already acquired a stony consistency, the exterior surface is quite 

 convex, and the margin of their apertures just out so they are 

 easily distinguished ; but by the progress of age their appear- 

 ance changes ; their free surface rises so as to efface the deep 

 depressions which marked originally their respective limits, and 

 to raise to the level of the surface the border of the openings. 

 The result of this is that the cells cease to be distinct, or even 

 distinguishable without, and, that the polypidom seems to be 

 formed of a stony continuous mass, in the substance of which are 

 excavated certain holes slightly widened interiorly, and disposed 

 in quincunx. 



