MUSEUM OF COMPARATIVE ZOOLOGY. 177 



size have been found by different expeditions, one by the " Challenger," 

 belonging to the DorididcB, being as large as an orange. All of these, 

 however, were of a peculiarly loose and gelatinous consistency. It 

 would seem as if a certain looseness of texture is required by the con- 

 ditions of great pressure which exist in the depths, in order to afford 

 that thorough permeation of the tissues by water necessary to equalize 

 the pressure. Whether this, as seems most probable, or the expansion 

 due to removal of the pressure on being carried to the surface, is the 

 cause of the looseness referred to, is uncertain, but that the deep-sea 

 animals of this group, as well as the fishes, exhibit such a state is cer- 

 tain. The shells almost without exception are extremely thin and light, 

 often reminding one of the delicate dwellings of some of the tropical 

 land snails ; to which a curious resemblance in form and texture may 

 frequently be noted. 



The colors of the abyssal shells are almost always faint, or delicate, 

 though often very attractive from their very delicacy. The iridescence 

 or pearly character of the shell, in many groups, is often of peculiar 

 brilliancy and beauty, and it seems as if the texture of many shells not 

 intrinsically pearly was nevertheless of such a character as to give out 

 a sort of sheen in the abyssal species which is wanting in their shallow- 

 water relatives, and may be compared to pearliness. 



While we do not find in any of the deep-sea species those sturdy 

 knobs and stout varices which ornament the turbinellas and conchs of 

 shallow water, and have made the great group of rock-purples, or Murices, 

 so attractive to collectors, there are nevertheless many abyssal shells 

 which have a delicate, and sometimes profuse sculpture, even more 

 elegant. The surface is frequently etched with a sort of shagreen pat- 

 tern, varied in detail and hardly perceptible except by a microscope, but 

 extremely pretty. In some the entire surface is adorned with profuse 

 arborescent prickles ; in others, pustulated with the most delicate shelly 

 blisters, systematically arranged, and which perish with a touch. In 

 most representatives from deep water of the family of scallops (Pecten), 

 the shell is as thin as a sheet of mica, its constituent prisms large 

 enough to be seen with the naked eye ; translucent, strengthened within 

 by delicate shelly riblets radiating from the hinge and often picked out 

 externally with delicate dots and splashes of orange, scarlet, or maroon. 

 Some of the family of top-shells {Trochidoe) are variegated with lovely 

 colors. In one form, those dredged in deep water by the Fish Com- 

 mission in the latitude of New York are stout, tall, and brightly varie- 

 gated with yellow and red-brown. In the specimens obtained from deep 



VOL. XII. — NO. 6. 12 



