2IO 



THE PIDDOCKS. 



wavy, concentric ridges and faint radiating lines. 

 The sculpturing is not sharply divided into two 

 sections as it is in the next species. Within each 

 valve, beneath the hinge, is a slender, curved pro- 

 jection, very narrow and delicate. On the outside, 

 just above the ligament, is a long, protecting plate, 

 with straight sides. This auxiliary valve, as it is 

 called, is curved in front and straight behind. The 

 shells of this species measure two and a half inches 

 in length, are widely gaping at the ends, and are of a 

 white color. This mollusk inhabits muddy flats 

 near the shores of San Francisco bay. 



Fig. 179 gives us a good idea of the shell of the 

 Rough Piddock, Zirphcea crispata, Linn., Zir-fe'-a 

 cris-pa'-ta. This fine borer is able to force a tunnel 



Fig. 179. 



into the hardest blue clay by means of the sharp, 

 rasp-like teeth, which are ranged in rows on the for- 

 ward part of the shell. Within each valve is a deli- 

 cate, spoon-shaped tooth or process, which joins the 

 shell just beneath the umbo. There is no accessory 

 plate over the hinge-area, but it is protected by a 

 membrane, and in front of the umboues the valves 

 are reflexed. 



The shell is thin, white and very hard. The 

 length is from two to four inches. This species is 



