CHARACTERISTIC DEEP-SEA TYPES. — MOLLUSKS. 63 
cious character of the mud forming the ocean floor would also 
tend to make motion through it slow and difficult. The deli- 
cacy of the shells, their extreme fragility and tenuity often re- 
minding one of the delicate dwellings of some of the tropical 
land snails, would unfit them for constant frietion and. collision, 
either from the motions of the animal itself or of the waters in 
which it lives. Swimming mollusks, such as the squids and 
cuttle-fishes, make an exception; but the deep-sea representa- 
tives of these groups are far softer and less muscular than their 
shallow-water allies. 
The colors of the abyssal shells are almost always faint, 
though often pretty. The iridescence or pearly character of the 
shell is in many groups of peculiar brillianey and beauty, and it 
seems as if the texture of the non-iridescent shells in the abys- 
sal species gave out a sort of sheen which is wanting in their 
shallow-water allies. 
We do not find in the deep-sea species those sturdy knobs and 
stout varices which ornament the turbinellas and conchs of shal- 
low water, and have made the great group of rock-purples, or 
Murices, so attractive to collectors; nevertheless many abyssal 
shells have an exquisite and rich sculpture, and their ornamen- 
tation is wonderfully delicate. There seems to be an especial 
tendency to strings of bead-hke knobs, revolving strie and 
threads, and delicate transverse waves. Many of the deep-sea 
forms, selected from all sorts of groups indifferently, have a row 
of knobs or pustules following the line of the suture and 
immediately in front of it. "Their surface is also frequently 
etched with a sort of shagreen pattern, varied in detail and 
hardly perceptible except by a microscope, but extremely pretty. 
In some the entire surface is profusely adorned with arbores- 
cent prickles; in others, it is covered with the most delicate 
shelly blisters, systematically arranged, which perish with a 
touch. 
Deep-sea mollusks may be understood to include all those 
living on the continental shelf, and in the abysses at depths 
where algæ do not flourish, the limit depending somewhat on 
the locality. Those living only above form the littoral fauna, 
which, roughly speaking, may be said to reach from the shores 
