" SHELL-FISH." IQQ 



It is difficult to get two shells that at all agree in the distribu- 

 tion of white and red, and even the two valves of the same 

 shell will differ widely in this respect. The interior, also, 

 exhibits characters sufficiently striking to prevent its mis- 

 identification. There is a broad flange below the hinge, 

 whereon are cut about twenty teeth, in two series. Immedi- 

 ately below the ligament is a smooth space, clear of teeth, but 

 these are arranged in a row of about ten on each side of this 

 space. As the shell grows the flange lengthens, and more 

 teeth are added to the ends of the rows farthest from the beak ; 

 but those nearest the smooth central space are being rubbed 

 down or absorbed at the same rate, so that the net increase is 

 about nil. The free edge of the valves, internally, has a series 

 of raised marks, like the tips of the teeth of a comb, and it is 

 from these the creature gets its name. The pallial impression 

 is much deeper than those of the muscles at each end of it, 

 and it is uninterrupted by any sinus. This shell is about two 

 inches in length. 



We must not omit to mention a group of shells that are 

 fairly common upon many shores, and are usually found 

 among the bucketful the children have collected. 



First of these is the bold Rayed Trough-shell (Mactra 

 stultoruni), more plentiful in the north than the south of 

 Britain. The various species of Mactra 

 are inhabitants of sand in deep-water, 

 but their shells are freely cast up on the 

 shore. These are smooth, except that 

 the annual periods of rest from shell- 

 making is plainly marked in deep con- 

 centric grooves. Like that of the Spiny- 



RAYED TROUGH-SHELL. ~ , i jv r , r ^ T l_ 



Cockle, the foot of Mactra can be ex- 

 tended and used like a finger, and also as a leaping-pole. 

 They are destroyed in great numbers by star-fish, and many 

 empty valves may be found with the clean round boring that 

 shows the animal fell a victim to the whelk. M. stultorum 



