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 friction 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 brilliancy 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-like knobs, revolving striae 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 algse 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 



