POWERS OF BURROWING. 339 



the following description and remarks, though applicable 

 I believe to all the species, are drawn up from that alone. 

 The cup (PL VIII, fig. 1 a', 1 c) is hardly ever regular 

 in outline, and is either slightly or very deeply concave ; 

 I have seen one, half an inch in diameter ; it is formed of 

 several thick layers of dirty white, translucent, calcareous 

 matter, with sinuous margins ; externally the surface is 

 very irregular, and is coated by yellow membrane presently 

 to be described. The innermost and last-formed layer 

 sometimes covers the whole inside of the cup, and ex- 

 tends a little beyond its margin all round ; but more 

 generally it projects beyond only one side, leaving the 

 other sides deserted. I have seen a single new layer 

 extending beyond the underlying old layers, as much as 

 one sixth of an inch ; and again I have seen a part of the 

 cup, as much as a quarter of an inch in width, deserted 

 and covered with serpulae. So irregular, however, is the 

 growth, that after a period an old deserted portion will 

 occasionally be again covered by a new layer, though of 

 course without organic adhesion. Again it sometimes 

 happens that the last-formed layer, remaining central, is 

 very much less than the older layers ; in one such instance 

 the innermost and last-formed layer (fig. 1 a) had a dia- 

 meter of only a quarter of that of the whole cup, in the 

 middle of which it was placed ; the cup thus tends to 

 become filled up in the middle. The cup, in its fully de- 

 veloped condition, is seated at the very bottom of the 

 cavity in the rock. From the aggregate thickness of the 

 several component layers forming the cup, the old and 

 mature animal rises a little in its burrow ; for instance, 

 the bottom of the cup in one specimen which I measured, 

 was -^ths of an inch in thickness. 



In a younger condition, before the animal has bored 

 down to the full depth, and whilst the cavity is only of 

 moderate diameter, the lower part of the peduncle, instead 

 of being attached to the inside of a cup, adheres to small, 

 irregular, nearly flat, calcareous discs, overlapping each 

 other like tiles (figs. 1, 2 a). They are placed one 



