MAEINE BIOLOGICAL STATION AT POET EKIN. 289 



from 20 to 50 fathoms give us the most varied bottom 

 deposits and the richest fauna. As a rale, the sand is 

 more or less mixed with mud, and as the bottom goes 

 deeper the amount of mud gets greater. When there is a 

 considerable admixture of mud with coarse sand, that 

 forms what is known to the trawlers as a " reamy " bottom, 

 and that is the ground upon which the sole and some 

 other fish are generally found spawning. 



Shells and other hard parts of animals play an important 

 part in the deposits at depths of about 20 fathoms and 

 upwards. In places the dredge comes up filled with 

 Pecten shells, dead and alive, chiefly P. operciilaris and 

 P. maximus. At other places the deposit is practically 

 composed of the shells of Pectunculus glijcimeris. These 

 and other shell beds form a rich collecting ground to the 

 naturalist, as they support an abundant and varied fauna. 

 Zoophytes and polyzoa are attached to the shells, and 

 these serve as shelter for nudibranchs and other small 

 mollusca, worms, and ascidians. On the whole the hete- 

 rogeneous deposits support a richer fauna than do the 

 homogeneous deposits, such as sand or mud, and it is 

 chiefly in the zone of depth we are now considering that 

 the heterogeneous deposits occur. 



4. The depths over 50 fathoms contain a pure dark 

 bluish grey mud, which is very tenacious, and sets when 

 dried into a firm clay. This is abominable stuff to dredge 

 in and to work with on deck. It chngs to everything that 

 touches it; it is almost impossible to see what is in it, and 

 to get the animals out of it uninjured ; it is too solid for 

 the sieves, and the hose can be played upon masses of it 

 almost indefinitely without dissolving it. The fauna of 

 this zone is, in our district, quite pecuHar and character- 

 istic. In its shallower parts, about 50 fathoms, it contains 

 great numbers of hving and dead Tarritella terehra, upon 



