occurring in Plymouth Harbour. 207 



Limnoria terebrans, or gribble. I may state as preliminary 

 to my observations, that the harbour of Plymouth is formed 

 by the junction of two Rivers, the Plym and the Tamar; the 

 mouth of the Plym on the east constitutes the harbour of 

 Catwater, about one mile long, which is occupied by mer- 

 chant ships ; the mouth of the Tamar, on the westward, forms 

 the harbour of Hamoaze, about 2j miles long, which is de- 

 voted chiefly to the Royal navy ; the confluence of the wa- 

 ters of these two rivers, forms the Sound, another harbour of 

 large extent, about three miles square, which is defended from 

 the open ocean by the breakwater. In this large space there 

 are many places favourable for the existence of the Teredo 

 and Limnoria, the latter of which is known to be most abun- 

 dant between high and low water mark. 



Within the harbour of Catwater, a large portion of land 

 was enclosed, about 20 years ago, by an embankment, about 

 one mile in length, for the purpose of forming a more direct 

 road to Plymouth ; and as it was necessary to make provision 

 for occasionally letting out the rain water, which might ac- 

 cumulate within, tunnels were made at different points through 

 the bank, which were closed by wooden doors, about 2 feet 

 square, opening outwards so as to prevent the ingress of the 

 sea during high water. In the course of a few years these 

 valves began to leak, so that in many instances the land was 

 overflowed by the sea ; on examination it was found that the 

 wood had become as it was thought rotten, and it was there- 

 fore renewed when necessary ; at length the constant dread 

 of inundation, and the injury sustained by so frequently open- 

 ing the bank, induced the proprietors to cause iron doors to 

 be substituted for wood ; about six months ago, I happened 

 to be present when one of the wooden doors, (one third of 

 which was destroyed) was taken out, and instantly discovered 

 that the wood which was called rotten, had been in fact eaten 

 by the Limnoria, thousands of which were at that moment to 

 be seen, and among them were four specimens of the Teredo ; 

 this wood is still in my possession. Thus it appears that in 

 two years, which is the time that this -door had been put in 

 the bank, apiece of elm, 2 feet square and 2 inches thick, had 

 been nearly one third destroyed by these animals ; and I have 

 not the least doubt but that the previous destruction of the 

 valves had been effected in the same manner, as I know that 

 elm may be kept much longer than two years under salt 

 water without manifesting any indications of decay. 



The next instance which presents itself of the appearance 

 of the Limnoria, was at the mouth of Catwater, where " the 

 Busy," revenue cruiser, has for several years occupied the 



Vol. II.— No. 16. n. s. s 



