98 



shells are chambered, by the interposition of thin hemispherical plates, 

 without any syphon, at a little distance from each other. He has also 

 ascertained, that these septse, not only are not always at equal distances, 

 but that sometimes they are fixed to the sides of the tubes, and com- 

 pletely preventing any communication between these chambers ; and 

 that, at other times, on the contrary, they are not adherent to the sides, 

 and may, of course, be easily removed. 



The two specimens which I possess, of this fossil, are only two small 

 fragments. In the one represented Plate VII. Fig. 3, is seen the na- 

 ture of the change which the shell here undergoes. Through more 

 than one half of its length the fissure is partially closed, evidently by the 

 approximation and augmentation of the denticular processes, with which 

 its sides are beset ; and, through the remaining part of its length, the 

 union of the sides of the fissure is complete. In the other specimen, the 

 fissure has been completely closed, a slight tubular excavation of the 

 outer surface showing where the fissure had formerly existed. No septae 

 exist in either of these specimens. 



In this shell, the fissure seems to fill up as the animal proceeds; the 

 sides of each of the chambers being filled up, perhaps, before the portal 

 is finally closed, by fixing the septa. 



