ON THE MARINE ZOOLOGY OK THE IRISH SEA. 329 



and YY shell concretions. The cliief conclusions we have arrived at so 



far are : — 



1. The most extensive shallow-water deposit is sand. In most 

 localities along the coast of Lancashire, Cheshire, and North Wales, from 

 the sea-shore out to the 10-fathom contour the bottom is formed of more 

 or less pure quartz sand. Occasionally in spots there are local patches of 

 stones, of shells, or of mud ; but these can generally be accounted for by 

 tidal or estuarine currents, by the entrance of fresh- water streams carrying 

 down alluvium, or by the presence of littoral or sub-littoral boulder clay. 

 These spots are all, however, of small area, and the great extent of the 

 bottom down to 10 fathoms is sand. 



2. Further out, however, between 10 and 20 fathoms, the sand becomes, 

 greatly mixed with mud, and much diversified by large tracts of shelly 

 deposits or by patches of gravel, and the fauna on the bottom also becomes, 

 much more abundant. In some spots, at about 20 fathoms, it is made up 

 over considerable areas almost entirely of ophiuroids (Ophiocoma nigra and 

 Ophiothrix fragilis), which fill the dredge haul after haul. At two localities, 

 off the Isle of Man, viz., along the east coast from Clay Head to St. Ann's 

 Head, and off the west coast between Contrary Head and Niarbyl, at 

 depths between 10 and 20 fathoms, are great nullipore deposits formed 

 of Melobesia and Lithoihamnion, which have a most characteristic appear- 

 ance, smell, and fauna. 



This area of the sea-bottom, from 10 to 20 fathoms, extends across 

 from the north of Lancashire to the Isle of Man, so that opposite Barrow, 

 for example, there is a wide extent of about 50 miles in length of sea-floor 

 at depths of not more than 15 or 16 fathoms (see section at foot of 

 Plate I.). 



3. Depths of over 20 fathoms are only found to the west, north, and 

 south of the Isle of Man (see Chart, Plate I.) ; and depths of from 20 to 

 50 fathoms give us the most vai'ied bottom deposits and the richest fauna. 

 As a rule 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 consider- 

 able 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. 



Siiells 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. opercidaris 

 and P. maximus. At other places the deposit is practically composed of the 

 shells of Pectuncuhis ghjcimeris. 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. 

 (_)n the whole the heterogeneous deposits support a richer fauna than do 

 tlie 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 bluisli 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 clings to every- 

 thing 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 



