20 Proceedings of Royal Society of Edinburgh. [sess. 
above described were tbe casts excreted by these creatures, which 
subsist on what nutriment they can pick up by triturating and 
passing the sand or mud through their bodies. In some of these 
animals the triturating apparatus takes formidable proportions, as 
in sea-urchins ; and it is probable that the sand found at low water 
owes its state of comminution largely to these animals and to worms, 
such as the ordinary lob-worm used for bait. When examining 
deep-sea clays in the “ Challenger ” I had observed the pellet 
formation, without, however, being able to refer it to any probable 
cause. Now, however, it became probable that the same causes are 
at work in deep as in shallow seas, and that the matter forming 
the bottom of the sea is being continually passed and repassed 
through the bodies of the numerous tribes of animals which 
demonstrably subsist on the mud and its contents. 
In the following season, 1879, I made an extended cruise through 
the greater part of the waters of the west coast of Scotland, visiting 
most of the deeper spots, and paying particular attention to the 
occurrence of coprolitic mouldings of the mud. Thus, on 16th 
June 1879, dredging in the deep part of the Sound of Eaasay in 
155 fathoms,* “a little mud came up. It was a fine gray clay, 
which effervesced with acids and smelled of H 2 S. On washing a 
quantity of it there remained the coprolitic masses and very little 
fine sand. There appeared to be a good deal of carbonate in a very 
fine state of division. There were very few shell particles visible, 
and the effervescence of what looked like flocculent clay was not incon- 
siderable.” At the time I explained this flocculent carbonate as 
having been produced out of the silicates of the mud by the ground 
animals forming sulphide of calcium, which was transformed into 
carbonate by the carbonic acid of the water. On the following 
day another haul was got in the same locality and with similar 
results ; it is noted that — “ Sticking to the outside of the bag were 
many legs of ophiurids, which will account for the coprolites.” 
When attention had once been paid to it, the coprolitic moulding 
of the mud, when of a suitable consistency, was found to be 
practically universal round our shores, f 
* From deck-book of Steam Yacht “ Mallard,” 1879. 
t Later, in the year 1886, when in charge of the expedition to survey the Gulf 
of Guinea in the steamship “ Buccaneer,” I found the same thing practically 
