DESCRIPTION OF SHELLS. 171 
same genus (Limopsis scalaris) is found at Barton, but neither at Bracklesham 
nor at Grignon. They both occur in Belgium, but in different localities. 
Iithodomus Deshayesti. (Tab. II. fig. 28.) 
Mytilus Lithophagus, Linn., is a recent shell, the animal of which corrodes holes 
in limestone*, and the fossil before us strongly resembles it ; but as its margins 
are not so parallel to each other and the sides are a little flattened, which make 
it less cylindrical than the recent shell, and as it does not appear to acquire so 
large a size, we think we are warranted to consider it a distinct species, and 
therefore name it after M. Deshayes, who appears to be the first author who has 
noticed the fossil distinctly. 
Our example was discovered in a mass of Siderastrea Websteri, into which it 
had bored ; Deshayes’s specimen had inserted itself into a Cerithium giganteum ; 
in both cases the masses were too small to have held a larger species, and belong 
to the same age. 
* That the holes in which this and several other genera of bivalved Mollusks, and a few other marine 
animals live, are corroded by a fluid emitted from the animal (possibly not a peculiar secretion for 
the purpose, but, in consequence of its position, highly charged with carbonic or some organic acid), 
is proved by the holes being shaped to fit the shells, as in Petricola, that is, being more or less oval in 
the section, and having a ridge opposite to the furrow between the beaks, so that the animals cannot 
turn round; those animals are also always found in calcareous stones or corals. It is in some cases, 
as in Gastrochena (page 164, Tab. II. fig. 27), observable that the fluid loaded with lime as it passes 
out of the cavity deposits a portion near the opening, forming a tube with a more or less perfect septum 
along it to suit and protect the passage of the tubes of the animal (see fig. 27 a). In Lithodomus Dac- 
tylus this deposit is made also upon the surface of the epidermis of the shell near the opening of the 
tube, and forms a kind of beak; the presence of an alkaline carbonate in the water may facilitate this 
process if the solvent be an organic acid. This substitute for shell by an earthy deposit not organized, 
is a manifest link between the animal and mineral kingdoms, and one of the many proofs a naturalist 
meets with, of a unity and reciprocality of design in the works of Creation, which only an infinitely 
good and wise Power could execute. Pholas, Teredo, and allied animals, actually drill tubes for their 
habitations by means of the toothed edges of their shells, which are renewed as they wear away (see 
‘ Mineral Conchology,’ vel. i. p. 230); the motion in the segment of a circle by which this is done, is 
produced by the aid of proper muscles attached to the ligulate processes within the shell, and to the 
sides of the foot; the alternate action of these muscles with the foot as a fulerum produces the osciilat- 
ing motion required, the foot shifting its place from time to time as the hole is enlarged. In this way 
the Pholas cuts not only into wood and chalk, but into sandstone whose texture is loose while wet ; 
and Teredo bores into wood, leaving the marks of the teeth, which marks are often visible in fossils. 
Saxicava rugosa, some species of Byssoarca and even of Pecten, &c., live in holes ready formed, and to 
which they are obliged to fit their shells, beiug only able to maintain an open passage for their tubes. 
