Although some of the characters of the Animal to 

 which this shell belongs, may resemble those of Tere- 

 bratula, yet the different mode of its attaching its shell to 

 marine substances, and the very different texture of the 

 shell itself, (resembling in many respects the harder 

 corals) seems a sufficient apology for Lamarck's placing 

 it in another family, along with Calceola, &e. The 

 strong resemblance of its upper valve to that of Orbicula, 

 (a Genus in which the animal attaches itself by a byssus, 

 or tendinous substance that passes through a fissure ia 

 the lower valve) has caused much confusion, which has 

 been encreased by Lamarck's forming a third Genus, 

 (Discina) of a species of Orbicula ; an error that has 

 been pointed out and corrected by Mr, G. B. Sowerby, 

 in the I3th Volume of the Linn. Trans., and in his owu 

 work, under the respective Genera. 



Only one recent s])ecies is known ; it inhabits rocky 

 shores and coral reefs, in temperate climates ; several 

 fossil species have been described. 



