260 Contribution to our knowledge of Pelagic Mollusca. [No. 4, 



itself at the surface, easier and more expanded than when compressed 

 in the depths below.*" 



Now those learned in matters of hydrography, say that the propa- 

 gation of light through water is not carried far below the surface. 

 Its influence at the depth of 300 feet is scarcely equal to the glimmer 

 of twilight, and below about 700 feet there is probably perpetual 

 darkness. f 



If this be so, may we not assume that the vertical range of those 

 species at least, against whom the letters D. 0. N. stand, (see table, p. 

 269) would be between 1 to 50 fathoms ? The following experience 

 regarding three of those species, serves to strengthen this view. On 

 the night of the 2nd of October, 1866, while rowing round Ross 

 Island — the head quarters of the Port Blair settlement at the Anda- 

 mans, — in an open boat I took to seaward of it H. quadridcntata, 

 H. uncinata, and II. limbata. By the charts I find the depth 

 thereabouts varies from 9 to 50 fathoms. 



Before referring to the table, I will give one or two extracts from 

 my notes during the outward voyages in 1860 and 1868, which I 

 think may interest the readers of the Journal. 



8.30 a. m., one or two Cymhulia, nothing until 

 12.30. p. m., two II. tridentata, with number- 

 1860. Dec. 7, less embryonic forms. 



S. Lat. 39° 55', 1.30. p. m., six H. trid, with Satpa. 

 E. Long. 42° 12', 4.30. p. m. Hydrozooids and Salpce. 

 Therm. 63°, 7. P. m. three H. uncinata, medium size. 



Sea 67°, 7.30 p. m. Dozens of Sagitta, imbedded in 



Countercurrent. myriads of red Entomostraca (Copepods). 



7.45. p. m. Bag full of red Entomostraca, Sagitta, 



and two H. uncinata, medium size. 

 8. P. M. Sudden and total disappearance of red 

 Entomostraca, succeeded by Salpa with yellow 

 nuclei, and some Medusa, in which were 

 imbedded numbers of Balantium australe. 

 9.30 p. m. Two or three Cleodora pyr -ami- 

 data of different sizes. 



* Some species, however, seem quite indifferent to solar influence. 

 f Petermanns Atlas. 



