380 ON THE FAUNA AND BOTTOM-DEPOSITS NEAR THE SO-FM. LINE 



gravel (47'6 per cent.) with fine sand (177 per cent.), and a little silt 

 (2'8 per cent.), the intermediate grades of gravel and sand being small. 

 Sample 103 is a mixture of gravel with mud, the proportion of silt here 

 rising to 17 per cent,, with only 10 per cent, of fine sand, the highest 

 percentage of silt in any sample examined from these grounds. 



For comparison with the figures in the table I have given in the last 

 column the texture of a sample of fine, sticky mud from Plymouth 

 Sound, which contains 70 per cent, of fine sand to 244 per cent, of silt. 



Tlic Organic Constituents of the Bottom-deposits. Mr. Worth reports 

 below on the relative amounts of organic carbonate of lime contained 

 in the different samples. The organic constituents from which this 

 carbonate of lime is derived consist, on the grounds under investigation, 

 almost entirely of the remains of molluscs, echinoderms, serpulids, and 

 polyzoa, and there is no example on the whole area of a " nullipore " 

 ground. It is especially noticeable in Grades II. and III. that the shell 

 material is composed very largely of entire small shells of lamelli- 

 branchs or of fragments of such small shells, and not of broken pieces 

 of large shells. Many of these shells show the clean circular perfora- 

 tion, such as is produced by the boring gasteropods. 



Sponge spicules are abundant in the finer grades. 



The Silt. Although constituting only a small percentage of the whole 

 material forming the bottom-deposit on the grounds included in the 

 present investigation, the silt is of great importance from the biological 

 point of view, since many animals living on the grounds obtain their 

 food by passing it through the alimentary canal and extracting such 

 nourishment as it contains. The silt from a freshly taken sample of 

 deposit, when shaken up with sea-water, forms a flocculent precipitate, 

 the greater part of which settles fairly rapidly. On the other hand, 

 when shaken up with fresh-water, a great deal of it remains in sus- 

 pension for a long time, and the water may have a muddy appearance 

 for several days. 



Microscopical examination of the fresh silt shows that a very con- 

 siderable portion of it is of organic origin, and the analysis and 

 examination of dried samples confirm this {cf. p. 386). Diatoms are 

 very numerous as well as foraminifera (for species recognised see 

 p. 442), and Coccoliths are invariably present in very considerable 

 numbers. 



Attention has recently been drawn to the presence of Coccoliths in 

 our coastal waters by Joly and Dixon (No. 57), who found them in 

 the precipitate obtained by treating a quantity of sea-water in a centri- 

 fugal apparatus revolving at a high rate of speed. They had, however, 

 been found by Behrens (No. 6) in samples of bottom-deposit from the 

 Baltic as long ago as 1873. 



