456 BULLETIN OF THE 



light of their environment, much may be hoped toward the elucidation of 

 great questions in Biology, and naturalists everywhere should strive to promote 

 deep-sea dredging as essential to the progress of Science. 



The circumstances which lead to the belief in the absence, more or less com- 

 plete, of competition between the members of the fauna, are not hypothetical, 

 but admitted facts. The " rain of food " from the sinking of weak or dead 

 surface forms is unquestionable, and the supply must, in the nature of things 

 as we know them, far exceed the demand, except in cases where physical factors, 

 such as currents, intervene to prevent the supplies from reaching the bottom. 

 It is illustrated by the absence or obsolescence of protective devices in deep-sea 

 species, without regard to systematic relations. The genus most abundantly 

 represented of all is Mangilia, which is devoid of an operculum, and the 

 diminution in size and solidity of this protective appliance is marked in all 

 the deep-sea gastropods. Nearly all the species are carnivorous by hereditary 

 tendency; those which are not, like the Trochidce and Docoglossa, become so by 

 necessity. Long reflection on the ornamentation of the shell in deep-sea gas- 

 tropods has led me to the conclusion that the characteristic features may be 

 accounted for on mechanical principles. The presutural rows of nodules so 

 characteristic of many abyssal gastropods (as in Daphnella limacinct) serve as 

 buttresses for the strengthening of the fragile and delicate structure which 

 bears them. Impermeable solidity is something not to be expected in organic 

 structures subjected to the immense pressures of the depths. Strength must 

 therefore be sought in corrugations of the thin shelly envelope, — stays and 

 buttresses of one sort or another. In shallow-water TrocJnJce the adult outer 

 lip is never reflected. Strength is secured by the internal thick wall of the shell, 

 reinforced at the aperture by ridges of nacre. In Turcicula and Gaza of the 

 depths, we have the margin strengthened by recurvature, as, for other reasons, 

 we find it in the equally thin land shells like Mesodon, Bulimus, or Cylindrella. 

 In the Unio and Melania of fresh-water streams, whose waters from the decay 

 of vegetable matter are overcharged with carbonic acid, we find a dense thin 

 greenish epidermis developed as a protection against erosion. In the depths, 

 where every portion of the shell must be permeated by the surrounding ele- 

 ment to equalize the external pressure, and where carbonic acid exerts its usual 

 malign influence on the limy parts of all organisms, we find a strikingly 

 similar protective epidermis developed in most unexpected places. Thus it 

 comes about, that in the Trochi, Pleurotomidse, and other characteristic abyssal 

 animals, we find those puzzling and remarkable counterparts of land and fresh- 

 water species of totally diverse groups, which have astonished every student of 

 the MoUusca who has seen them. 



But it is necessary to close these observations, the fruit of more than eight 

 years' study of these fascinating organisms. I do so with the sincere expres- 

 sion of my appreciation of the kindness and patience with which Prof. Agassiz 

 has seconded all my endeavors, and with hearty thanks to the numerous friends 

 and correspondents to whose courtesy and learning much of such value as this 

 Report may possess is justly due. 



