2 
Although some of the characters of the Animal t& 
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, &c. 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 in 
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 13th Volume of the Linn. Trans., and in his own 
work, under the respective Genera. 
Only one recent species is known ; it inhabits rocky 
shores and coral reefs, in temperate climates ; several 
fossil species have been described. 
