164 I'LV.MOUTII MARINE INVEIITEBKATE FAUNA. 



Dredging and Trawling Grounds of the Sound. 



The greater part of Plymouth Sound consists of comparatively 

 shallow water (4-6 fms.) with a bottom-deposit of fine muddy sand. 

 Winding through this is a channel of much deeper water, which repre- 

 sents the old river-bed of the Tamar. After leaving the Hamoaze (the 

 estuary of the Tamar) this deep channel turns northwards until it 

 strikes the northern shores of the Sound close to Millbay, wliere, bend- 

 ing sharply to the eastward, it attains a depth of 23 fathoms. The 

 channel continues to run eastwards, keeping within a short distance of 

 the shore but gradually diminishing in depth until it reaches the 

 Mallard Shoal, where, after passing southward between this and the 

 Winter Shoal, it becomes lost in the shallow water forming the central 

 part of the Sound (4-6 fms.). It is in this cliannel, and in the two 

 channels at the eastern and western etids of the Breakwater, that stony 

 ground suitable for dredging occurs. Tlie greater part of the rest of 

 the Sound, having a bottom of fine sand and mud, is better worked 

 with trawls. The principal grounds referred to in the records are the 

 following : — 



Millbay Channel (14-23 fms.). The deep channel off Millbay is 

 one of the most productive dredging grounds in the Sound. The 

 dredge brings up masses of stones of varying size (chiefiy limestone), 

 with a few shells, all free from any growth of red or brown seaweeds. 

 The stones are generally covered with a good deal of brownish mud, 

 and are much honeycombed by the boring sponge Cliona celata, and by 

 Saxicava rugosa, Folydora ciliata, and Dodecaceria concharum. Sponges, 

 compound Ascidians, Antennularia antennina, and small Hydroids and 

 Polyzoa are numerous ; a great variety of small Polychtetes, more 

 especially Phyllodocids and Syllids, are hidden amongst the cavities 

 on the surface of the stones, and masses of tubes of Filograna implexa 

 are frequent. 



The deepest part of the channel, which forms a deep pit or hole, is 

 remarkable for the abundance of Antedon bifida, the dredge often 

 coming up half full of these Echinoderms. From the sides of the pit 

 Tv.hvlarin indirisa is often obtained in quantity. 



Asia Shoal. Another productive dredging ground lies along the 

 northern edge of the Asia Shoal, which is really the southern margin 

 of the deep channel. The water is here shallower (5-7 fms.) than 

 in the Millbay Channel, the stones are chiefiy the shales and grits of 

 Drake's Island, instead of the honeycombed limestones found at Mill- 

 bay, and they are generally covered with more or less mud. Eed sea- 

 weeds grow on them in small quantities, and large Hydroids, especially 

 Antennularia antennina and Tahnlaria indivisa, are often abundant. 



