228 REPORT ON ALBATROSS MOLLUSCA DALL. 



as is seen in many fresh-water species; and sometimes of a beautiful 

 rich dark chestnut-brown, a color .also not rare among laud and fresh- 

 water species. The most common pattern when any exists is that 

 formed by squarish dark spots, which occasionally become fused into 

 bands. Among the archibenthal species found in depths from 100 to 

 300 fathoms this pattern of brown squarish spots arranged in spiral 

 series is notable in such forms as Scaphella junonia, Aurinia dubia, 

 Halia priamus, Conns mazei, etc. Instances of the green epidermis are 

 afforded by the various species of Nuculidw, Turcicula, and Buccinidce. 



The thick and solid layers of aragouite, of which maiij' shallow- 

 water species are chiefly built up, are represented in deep-water forms 

 by much thinner layers, while the nacreous layers are, if not more solid 

 in ab3ssal shells, at least more brilliant and conspicuous, perhaps be- 

 cause less masked by aragonitic deposits. A very large proportion of 

 the deep-water shells are pearly and derive their beauty from the bril- 

 liance of their nacre. 



In the matter of sculpture the mechanical effect of the pressure op- 

 erates against the development of weight and thickness in benthal 

 shells since the whole must be permeable. It is probable, too, that the 

 soft and sticky character of the abyssal ooze would put the possessor of 

 an unusually heavy shell at a considerable disadvantage in getting 

 about on the bottom. Any impermeable shelly structure on the ocean 

 floor would have to be strong enough to sustain without crushing a 

 weight hardly less than that borne by the rail under the driving-wheel 

 of an ordinary locomotive. It is sufficiently obvious from a mere state- 

 ment of the case that none of them can be impermeable. 



The heavy knobs or arborescent varices of shallow-water Murices are 

 represented in their deep-water congeners by extremely thin and deli- 

 cate spines and slender processes. These are probably all reminis- 

 cences of shallow-water aucestors, as it is difficult to imagine any cause 

 which iu the abysses would lead to a development of such defenses de 

 novo. 



The sculpture most usual on deep-water shells is of a kind which 

 serves to strengthen the structure, much like the ridges which give 

 rigidity to corrugated-iron work, or the curves used by architects in 

 wrought-irou beams. Spiral or longitudinal hollow riblets, a transverse 

 lattice work of elevated laminae such as are developed for similar rea- 

 sons on the frail larval shells of many gastropods, a recurvature of the 

 margin of the aperture in forms which in the Litoral Kegion never de- 

 velop such recurvature — these are instances iu point. 



Besides these there are small props and buttresses developed which 

 serve the same purpose of strengthening the frail structure at its points 

 of least resistance. Such are the garlands of little knobs so commonly 

 found in front of the suture in abyssal shells of many and diverse 

 groups. 



It is not intended to suggest that the methods above indicated have 



