198 BY THE DEEP SEA. 



and outward currents the inward bringing both oxygen and 

 food, the outward carrying off carbonic acid gas and other 

 waste. 



The large, thick, coarse-looking mussel shells we occasionally 

 find on the sands, measuring five or six inches in length, belong 

 to a different genus, and are called Horse Mussels (Modiola 

 modiohis). The valves in question may not have come far, for 

 the species occurs in sand and mud as near as low-water. 

 But it will not be found moored to rocks and weeds by its 

 byssus; instead, it burrows and weaves its enormous byssus 

 into a nest with sand and gravel mixed up with the threads. 

 They are said to be coarse and unpleasant tasting, so that 

 they are not used as food, and hence the name, Horse-mussel; 

 the prefix, horse or dog, before a popular name for animals or 

 plants, denoting its worthlessness as food, the sole criterion of 

 worth to the popular imagination being found in the answer to 

 the query, " Is it good to eat?" 



A very handsome shell, as well as a common one, is the 

 Comb-shell (Pectunculus glycimeris], whose thick round valves 

 may be found rolling on the beach, where they 

 have been washed up from the zoophyte ground 

 in deep-water. It is very variable in its mark- 

 ings, and yet there is so strong a family likeness 

 running through all its variations, that there is 

 not the slightest difficulty in its identification. COMB-SHELL. 

 To the touch the exterior is quite smooth, though 

 not glossy, but examined with a lens it will be found to be 

 covered with very fine and regular lines running from the beak 

 to the opposite edges. So fine and close-set are these, that a 

 line an inch long, drawn across them at right angles, will cross 

 about ninety of them. The valves are more or less covered 

 with a colouring of rusty red, relieved by numerous long sharp 

 wedges of white. These are on parts or the whole of the 

 shell, sometimes so plentiful that there is no room for solid 

 masses of the red colour, and it only shows in zigzag lines. 



