PHYSICAL FEATURES OF THE IRISH SEA. II 
3. Depths of over 20 fathoms are only found to the west, north, and south of the 
Isle of Man; and depths of from 20 to 50 fathoms give us the most varied 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 
considerable admixture of mud with coarse sand it 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. opercularis and P. maximus. At other places the deposit is practi- 
cally composed of the shells of Pectunculus glycimerts. 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 heterogeneous 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 clings 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 peculiar and characteristic. In its shallower parts, about 50 
fathoms, it contains great numbers of living and dead Turritella terebra, upon many of which 
are attached one, two, or three specimens of the little red anemone, Sagartia expansa, In its 
deeper parts, up to 80 fathoms, are found Calocaris macandreea, Hyalinecia tubicola, a small 
Lumbriconerets, Panthalis oerstedt, Lipobranchius jeffreysit, Brissopsis lyrifera, Amphiura chiapit, 
and Jsocardia cor. Numbers of large sausage-like muddy tubes, formed of stratified layers of 
interlacing threads of mucus in which the mud particles are closely entangled, are brought up in 
the dredge. These we have proved to be the tubes of Panthalis oerstedi, and the living annelid 
has several times been found in the tubes, but most of those we dredge up are empty, and the 
tubes are certainly far more numerous than the worms. Apparently the explanation is that the 
Panthalis forms a tube as it lies in the mud, and then when it moves away leaves its tube 
behind it (one can scarcely imagine the animal dragging such a tube through this tenacious 
deposit), and after a time forms another in a new situation. 
These are the leading conclusions we have come to so far in regard to the distribution 
of sub-marine deposits in our area. Two further questions now present themselves : first, the 
biological one—the effect upon the fauna; and secondly, the geological one—the origin of the 
deposits. In regard to the importance of the nature of the bottom to the animals living upon 
it there can be no doubt. Probably the nature of the deposit is the most important of the 
various factors that determine the distribution of animals over the sea-bottom within one 
zoological area. It is certainly more important than mere depth; a muddy bottom will 
support a similar fauna at 10 fathoms in one place and at 50 fathoms in another. Probably 
the most important influence in the environment of a lower animal is its food, and once 
beyond the narrow sub-littoral zone in which alge flourish—and to which, of course, certain 
phytivorous animals must be restricted—it is probably chiefly the nature of the bottom which 
