POWERS OF BURROWING. 337 



^th of an inch in diameter. In the lowest part of the 

 peduncle these scales, after each fresh exuviation, are ap- 

 parently soon worn entirely away by the friction against 

 the sides of the cavity ; hence in most specimens this part 

 of the peduncle is quite naked. This same part, how- 

 ever, is furnished with nail- or rather star-headed little 

 projections of hard, yellow, horny chitine (fig. 3 e). The 

 star on the summit seems generally to have about five 

 irregular points ; one star which I measured was ^th of 

 an inch in total width, the foot- stalk being only ^th of 

 an inch in diameter ; the whole projected gj^ths of an inch 

 above the surface of the peduncle ; from the footstalk a 

 fine tubulus runs through the membrane to the under- 

 lying corium. These star-headed little points are often 

 much worn down; in one specimen which was on the 

 point of exuviation, there remained, in the lower part, 

 close above the basal calcareous cup, only some hard, 

 smooth, yellow, little discs, on a level with the general 

 surface of the membrane, — these being the intersected or 

 worn down footstalks, with every trace of the calcareous 

 beads gone. But in this same specimen, under the old 

 peduncular membrane, there was a new one, studded with 

 the usual circular calcareous beads, slightly unequal in 

 size, generally about ^th of an inch in diameter, and 

 each furnished with a tubulus ; but as yet none of the 

 star-headed points of chitine had been formed. I believe 

 that these latter are developed from the tubuli leading to 

 the calcified beads, and, therefore, are formed directly 

 under them. In Z. cauta the lowest scales on the peduncle 

 are a little larger than in L. dorsalis, giving a frosted 

 appearance to it, and all of them are serrated (fig. 3 d) 

 round their entire margins. Generally only the scales in 

 the uppermost, or in the three or four upper rows are 

 serrated, and this only on their arched and protuberant 

 lower margins. The state of the serrated edge varies 

 extremely in the same species, from elongated conical 

 teeth to mere notches, according to the amount of wear 

 and tear the individual has suffered since the last period 



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