LITHODOMtJS. 



sortent plus. Une fois qu'ils y ont penetres^ leur byssus 

 lie prend plus d'accroissement/' We cannot imagine 

 that this remark has been made from actual observation, 

 because we believe it to be contrary to the nature of the 

 animal to be at one time attached by a byssus, and not 

 at another; and, moreover, we have ourselves seen 

 Lithodomi not more than one- eighth of an inch in length, 

 in as completely formed perforations as the fuller grown 

 specimens : the animal itself has never come under our 

 observation, but as far as we dare guess from circum- 

 stances in the shells^ and the calcareous beaks which 

 they often form at their smaller end and outside the 

 epidermis, they must approach more nearly in form to, 

 those of the Pholas, Saxicava, Petricola, and other tere- 

 brating animals than to those of the MytilL 



Shell transverse, regular, equivalve, elongated; when 

 the valves are closed, cylindrical; both ends rounded; 

 anterior end very short; covered on the outside with a 

 strong, horny, generally dark brown epidermis. Um- 

 bones anterior, scarcely prominent. Hinge linear, with- 

 out teeth. Ligament linear, internal; a very small 

 portion to be seen externally. Muscular impressions 

 two, indistinct; anterior very small, posterior rather 

 large, oblong. 



We have said above that the general form of the 

 Lithodomi, when the valves are closed is that of an elon- 

 gated cylinder, but we might add that the hinge line 

 generally makes a more or less obtuse angle with the 

 posterior margin, and that the perforation they form in 

 stones, madrepores, &c. is very nearly of the same shape 

 as the shell; it being considered, moreover, that these 

 shells are covered with a smooth, horny epidermis, it 

 will appear impossible that they should form the cavity 

 in which they dvt^ell by terebrating : is it not then reason- 

 able to conclude that they are possessed of a solvent 

 fluid, w^hich, in combination with the sea water, produces 

 the cavity by a chemical action. 



The type of this Genus is the Mytilus Uthophagv^s of 

 authors, which is very common in the Mediterranean, 

 West Indies, and in all Coral rocks. Of the recent species 

 that are known, and which are very few, we have not any 

 on our own coasts. 



The fossil species, which abound in the great Oolite, 



