Genera of the ISTorth American Palaeozoic Bryozoa. 425 



. " We see, also, that that which is considered generally as being 

 the body of these polyps, constitute in reality only a small por- 

 tion of it, and consists of little but the digestive, and probably 

 breathing organs, of these little animals. 



" The tegumental bag, freed from its carbonate of lime, seems 

 to me formed of a tomentose membrane covered, particularly 

 without, with a multitude of cylindrical filaments, disposed per- 

 pendicularly to the surface, and pressed close to one another. It 

 is in the space left between these fibers that the calcareous mat- 

 ter appears to be principally deposited, for if we examine, with 

 the microscope, a transverse cut of the polypidom in its natural 

 state, we distinguish in it an analogous conformation, the exter- 

 nal wall of the cells being not composed of layers, but rather of 

 cylinders or irregular prisms placed perpendicularly to its 

 surface. 



" As to the operculum, which serves to shut the entrance of the 

 tegumental cell of the Eschara^ when the animal is wholly con- 

 cealed in it, it is but a labial fold of that which we may call the 

 skin of the polyp, and of which the projecting margin has 

 acquired a horny consistence, whilst that portion continuous with 

 the general envelope preserves sufficient softness to remain 

 flexible, and to obey the action of the muscles whose tendons are 

 inserted in its thickness. 



" The changes which we have indicated above in the external 

 formation of the cells of the Eschares are not the only ones 

 effected by the progress of age in the stony integuments of these 

 zoophytes. The form of their opening is modified considerably, 

 as may be seen by the figures which accompany this memoir ; the 

 sinus or emargination situated under the operculum disappears by 

 degrees, and their interior cavity becomes filled up so as not to 

 occupy more than about the quarter of their original diameter. 

 This thickening changes even a little the general appearance of 

 the polypidom ; for as it is more considerable in the cells situated 

 farthest from the extremities of the branches, it results that 

 these, at first almost flat, become more and more cylindrical. 

 Lastly, it is not without surprise that we have seen these same 

 cells when they arrived at extreme old age, lose altogether the 

 opening from which the polyp extended its tentacula. In fact 

 the margins of this opening, swelling more and more inwardly, 

 64 



