loo Shell Life 



byssus, and on the lower margin there is an indentation 

 whence this issues. There is a long tooth on each 

 side of the hinge-line, those of one valve interlocking 

 with those of the other. The animal agrees with its 

 shell in being almost transparent. The rear portion 

 of the fringed mantle forms a brief tube for the out- 

 ward current of water, and the foot is large and 

 powerful. From 8 to 100 fathoms deep is the range 

 of this species on all our shores, but not on the sand 

 or mud affected by its allies. It has a more remark- 

 able habitat, being found only on living Sea Urcliins, 

 and chiefly on the spines of the Heart-Urchins, sucli 

 as Spotangus ijurpiireus, EcJdnocardiuni cordatum, 

 and Bryssiis lyrifer. To these spines it is attached 

 by the coarse byssus-threads, and no doubt it selects 

 this strange position in order to share the food of the 

 Urchin. The Heart - Urchins, it may be mentioned, 

 bury themselves in the sand, and appear to exist by 

 continually swallowing it and digesting out the 

 organic matter mixed with it. The Urchin's mouth is 

 a shovel-shaped aperture on the lower surface, and 

 Montagu's-shell is attached to a spine in front of it 

 where the indraught of sand is likely to bring minute 

 food - particles to the mollusk. The two - toothed 

 Montagu - shell (M. hidentata) is less oval than 

 the last, approaching nearer to S([uare ; flatter, 

 and much less transparent ; whiter in colour. Tlie 

 teeth in the riofht valve are shorter and broader than 

 those in the left. The mantle has its margin fringed 

 with bright silvery teeth, and forms a very brief tube 

 behind. The large broad foot is somewhat triangular 

 in outline. It may commonly be found on all our 

 coasts, in water from 10 to 70 fathoms, nestling in 



