148 Pelagian Molhisca collected on a [no. 2, new series, 



presence. As most of them are crepuscular or nocturnal animals, 

 they rarely if ever make their appearance in the bright sun light, 

 but no sooner does the twilight approach, than different species 

 successively come to the surface, and they may then be readily 

 captured in the towing net. 



I have appended a chart* showing the different localities where 

 we met with any Molluscs, as I conceive that by collecting and 

 comparing the experience of different voyagers, not only may ouir 

 acquaintance with the nature of these animals be greatly extended 

 but it may add a link to our imperfect knowledge of the geogra- 

 phical distribution of animal life, a subject, the importance of which, 

 can hardly be overrated. 



It is only by comparing a sufficient number of such charts that 

 the facts they convey can be made practical use of ; there are ob- 

 vious reasons why one voyager may not meet with success in lati- 

 tudes where another may have been eminently fortunate. Alterna- 

 tion of season, change of temperature, or currents prevailing at 

 different times and places, must greatly influence the movements of 

 these creatures and may often occasion their involuntary migration: 

 again Pteropods and other MOLLUSCA may at times be pre- 

 sent in abundance without the voyager being able to capture a 

 single specimen. This happens when the speed of a ship exceeds 9 

 or 10 knots an hour, a circumstance of frequent occurrence on the 

 present voyage, as our ship, an American built clipper, sometimes 

 attained a speed of 15 and 16 knots an hour, when any attempt 

 •with the towing net involved immediate destruction of the net, if 

 not loss of the towing line. 



The first part of our voyage, which we commenced in the latter 

 part of March, was very stormy, and therefore peculiarly unfitted 

 for the prosecution of any researches in Natural History. We could 

 however watch the " stormy Petrels" following the ship for stray 

 garbage, while an occasional shoal of porpoises or other Cetacea. 

 varied the scene by their fantastic gambols; at night the wake of the 

 ship and the crests of the waves shone like luminous vapour, with 



Plate No. XII. 



