66 REPORT — 1841. 



Mr. Strickland abstained from naming them before he had communicated with Mr. 

 Stutchbury. The best known type of the genus is C. Listen ( Unio Listeri, Sow. Min. 

 Con.). Mr. Strickland also exhibited the unique specimen of a dragon-fly's wing 

 from the lias, the property of Mr. Gibbs of Evesham, figured in Mag. Nat. Hist. 

 1840, p. 301. 



On the Geological Changes produced by the Saxicava rugosa in Plymouth 



Sound. By W. Walker. 



The Saxicava rugosa appears to be the prevailing perforator of the limestone rocks ; 

 and it is the author's opinion that these operations have been carried on during such 

 long periods as to "destroy rocks, and make deep water where shoals previously ex- 

 isted." The different places in which the perforations had been extensive, or might 

 be most advantageously examined, were described. Tlie blocks of Portland stone, 

 to which the buoys were formerly attached, were in two or three years punctured 

 on the surface and also deeply perforated by the Saxicava ; and in the sea-walls of 

 Devonport dockyard, also of Portland stone, below the low-water level of spring 

 tides, the stone is honeycombed and frittered away. Between the Devil's Point and 

 Mount Edgecumbe the channel is 200 feet wide, and three or four times deeper than 

 in the Sound. In using the diving-bell to cut away rocks in this channel, in order 

 to form the sea-walls of the Royal William Victualling Yard, the limestone was found 

 much perforated, and rocks brought up by the trawl from twenty and twenty-five 

 fathom water were also perforated. The sides and bottom of this channel are of 

 limestone, and the rapidity of the current keeps its suiface clear ; wherever this is the 

 case, throughout the Sound, the bottom is found perforated by the Saxicava. The 

 author then proceeds to show, that throughout the Sound there is a greater depth of 

 water over the limestone rocks than over the red sandstone, and he attributes this 

 degradation to the boring of Saxicava. The rocky shoals lying southward of a Hne 

 drawn from Mount Edgecumbe through Drake Island to Mount Batten are com- 

 posed of red sandstone, and lie from twelve to seventeen feet beneath the level of low- 

 water spring tides. The depth of water over the anchorage, within the Breakwater, 

 varies from twenty-seven to thirty-six feet ; but immediately on passing the boundary- 

 line of the sandstone, northward, the water becomes deeper, and where the rapidity 

 of the tidal current prevents deposits of mud or sand, the depth varies from fifty to 

 one hundred and twenty feet over the limestone . 



The limestone hills around Plymouth were next described ; and it appears that 

 their elevation is generally less than that of the surrounding sandstone. These lime- 

 stone heights at Stonehouse, Plymouth, Mount Batten, and Oreston, are nearly of 

 the same elevation ; raised beaches, rounded pebbles, and water-worn surfaces, fur- 

 nish evidence of their having been once under water. Near the north-west corner 

 of Drake Island is a small portion of limestone on a level with the ordinary high 

 water, bearing traces of the perforations of Saxicava ; and the rocky cliffs near Ply- 

 mouth Citadel, from the low-water mark of spring tides, up to fifteen or twenty feet 

 above high- water level, bear evidence of the same ravages ; and at fifteen or twenty 

 feet above the sea is a conglomerate mass of pebbles, sand and shells, and some 

 limestone pebbles, perforated by Saxicava. At low-water mark are the animals 

 in their holes, higher up their empty shells are seen, and above high water their 

 perforations only are to be found. In the limestone in Hoe lake the author very 

 recently found perforations by the Pholas, one hundred feet above the level of low- 

 water spring tides. The situation was protected from atmospheric influence by a ■ 

 coating of earth and vegetation. From these circumstances the author inferred that 

 all the limestone rocks around Plymouth were under water within the period during 

 which the Saxicava was the great agent in the destruction of the rocks. In some cases 

 the rocks are protected from these ravages by a coating of Balaiii, &c., which cover the 

 rock ; and in other places deposits of mud and sand are formed over the rocky bottom, 

 and there the operations of Saxicava and Pholas necessarily cease. Since the Break- 

 water was erected, the water over the rocks near the Citadel has been diminishing in 

 depth, from the accumulation of mud and sand, and an anchorage is forming where 

 nothing but rocks previously existed. In conclusion, the author refers to the ope- 

 rations of boring shells on the mole of Castel a Mare in the Bay of Naples, and on 

 other artificial structures, and suggests to engineers employed in the erection of pub- 



