PRESIDENTIAL ADDRESS. 17 



In the matter of sculpture the mechanical effect of the pres- 

 sure operates 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 

 statement 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 ex- 

 tremely thin and delicate spines and slender processes. These 



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are probably all reminiscences of shallow water ancestors, as it 

 is difficult to imagine any cause which in 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 iron beams. Spiral or longitudinal 

 hollow riblets, a transverse lattice work of elevated laminae 

 such as are developed for similar reasons on the frail larval 

 shells of many gastropods, a recurvature of the margin of the 

 aperture in forms which in the Litoral region never develop 

 such reCurvature ; — these are instances in point. 



Beside these there are small props and buttresses developed 

 which serve the same purpose of strengthening the frail struc- 

 ture at its points of least resistance. Such is the garland of 

 little knobs so commonly found in front of the suture in 

 abyssal shells of many and diverse groups. 



