18 BIOLOGICAL SOCIETY OF WASHINGTON. 



It is not intended to suggest that the methods above indi- 

 cated have not been developed also in shallow water forms and 

 for similar reasons. The distinction which I would point out 

 is that in Lvitoral species, as a rule, these devices are subsidi- 

 ary to the much simpler course of strengthening the shell by 

 adding to its thickness. In the abyssal forms, for reasons 

 already explained, this mode is not practicable and conse- 

 quently we have the one without the other. 



The operculum is generally horny in abyssal mollusks, fre- 

 quently disproportionately small, compared with that of con- 

 generic litoral species, and in a remarkably large number of 

 cases is absent altogether. 



The genus most abundantly represented of all is Mangilia, 

 which is entirely without an operculum, and affords a conspicu- 

 ous example of the obsolescence of protective devices, origi- 

 nally acquired in shallow water, resulting from long residence 

 in the deeps. 



In the Unio and Melania of fresh water streams and the 

 pondsnails of our lakes and ponds, the waters of which 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 deep sea where every por- 

 tion of the shell must be permeated by the surrounding element 

 to equalize the external pressure, and where carbonic acid 

 exercises its usual malign influence on the limy parts of all 

 organisms, we find a strikingly similar protective epidermis de- 

 veloped in most unexpected places. Thus it comes about that 

 in the Troc/ii, Pleurotomidce and other characteristic abyssal 

 animals we find those puzzling and remarkable counterparts of 

 land and fresh water shells which have astonished every student 

 of the mollusca who has seen them. These deep water species 



