DREDGING IN THE RIVER CROUCH. 85 



The Other valve has an opening into which the " stopper " fits, and 

 by which it is attached to stones or other shells. On the south 

 coast it occurs abundantly of considerable size, on the shell of the 

 large Scallop {Fectcn maxiinus) with both the valves moulded to the 

 conformation of the ribs of that toothsome mollusc. Some frag- 

 ments of wood were brought up riddled through with the borings of 

 a species of ship-worm. Teredo tiavalis (not really a zvo}'m, but a true 

 mollusc allied to the rockborers, Fholas, &c.) ; but in most cases 

 the brittle calcareous lining only was to be seen. On breaking the 

 wood up carefully, I was able to obtain the boring valves and a few 

 of the pallets, which enabled the species to be determined. The 

 valves are exquisitely sculptured in series of lines, and the pallets are 

 embedded nearly to the cusped tips, close by the long siphons. 



The use of these pallets, which vary very greatly in the different 

 species, has never been clearly ascertained ; but they are probably 

 used to close the tube and protect the mollusc when the siphons 

 are retracted. 



A few good-sized specimens of Modiolaria marmorata, and the 

 larger form, M. nigra, were taken ; and a large species which is called 

 the " Horse mussel " {Mytilus modiolus), which has a more hand- 

 some shell, well distinguished from the common mussel, not only 

 by the rich brown colour and thickish epidermis, but by the umboes 

 of the shell not being on the apex. 



One dead valve of Cardium echinatum (the Horse Cockle), 

 showed that this exists in the Crouch, and we obtained many live 

 specimens of the small C. exiguuvi. 



Another species of Tapes was also found, T. aureus, so called 

 from the yellow colour of the interior of the shell ; but the shape 

 also is very different from the other species, the contour being more 

 wedge-like in form. 



Some specimens of Scrobicularia alba came up ; but, strangely 

 enough, S. piperita, which is larger, and the shells so common around 

 the coast at Walton on-Naze, Harwich, Felixstowe, &c., did not seem 

 to be present — perhaps it is more purely marine than its smaller 

 ally ; or its absence may be due to the clearing of the bottom of the 

 river for the oyster culture. We also got two species of Sandgapers, 

 Mya arenaria and M. iru?icata. One of the latter, which I kept 

 alive some time, measured, when the siphons were fully extended, 

 5 1 inches. 



Of the Gastropods (Univalves), we took two dead shells of 



