5 



FAMILY 6. LITHOPHAGA. 71 



growth, from its habit of terebrating*, it is the well-known Saxicava 

 rugosa of authors. 



The shell of Saxicava may be described as being transverse, irregular, 



* In order to show the diversity of opinion that still exists as to the means employed by 

 many of the Mollusca for terebrating or boring, we extract the following elaborate report, 

 from the ' Athenreum,' of an interesting discussion that took place on the subject in the 

 Geological Section at the recent meeting of the British Association at Plymouth. 



" Mr. W. Walker observed, that the Saxicava rugosa appears to be the prevailing perfora- 

 tor of the limestone rocks ; and that these operations have been carried on during such long 

 periods, as to ' destroy rocks and make deep water where shoals previously existed.' The 

 blocks of Portland stone to which the buoys were formerly attached, in two or three years 

 were 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 honey-combed and frittered away. At low-water mark are the animals in 

 their holes, higher up their empty shells, and above high water their perforations only are to 

 be found. From these circumstances it appears, that all the limestone rocks around Ply- 

 mouth 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 Balani, etc., 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 neces- 

 sarily cease. Since the Breakwater was erected, the water over the rocks near the Citadel has 

 been diminishing, from the accumulation of mud and sand, and an anchorage is forming 

 where nothing but rocks previously existed. 



" Mr. De la Beche stated, that the raised beaches and attendant phenomena at Plymouth 

 had been first ascertained many years since by Mr. Hennah. The perforations were made 

 apparently by several other animals as well as Saxicava, and he considered these perfora- 

 tions chiefly destructive by exposing a greater amount of surface to the action of other 

 agents, such as carbonic acid. 



" Dr. Buckland remarked, that with regard to the specimens of perforated rocks, some 

 from Mount Batten were decidedly neither the work of Saxicava nor Pholas, but precisely 

 resembled some detected by Mr. Greenough in Boulogne, which were the work of common 

 garden-snails (Helix aspersa). 



" At Tenby, Mr. Phillips and Dr. Buckland had observed the same kind of perforations ; 

 and Mr. Sopwith had noticed them as occurring commonly in Northumberland, on the under- 

 side of overhanging shelves of mountain-limestone. The holes made by the Pholas and Saxi- 

 cava are smaller at the aperture, and increase inward as the shell grows, always assuming its 

 shape. Those made by the snails are largest in the aperture, irregular in form and direc- 

 tion, so as often to trench upon one another. Dr. Buckland attributed the perforations of the 



