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 septae, not only are not always at equal distances, 
but that sometimes they are fixed to the sides of the tubes, completely 
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 septse 
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. 
