THRACIA. 33 



nullipore bottom at various depths ; but a British species 

 of Thracia (T. distorta) prefers a more secluded habitat, 

 and occupies the deserted holes made by Saxicava rugosa 

 in limestone, or other rock-cavities, as well as tufts of 

 Corallina officinalis. 



Genus THEA'CIA* Leach. PI. II. f. 2. 



BODY oval : tubes separate. 



SHELL oval, nearly equilateral, rather thin, having a tuber- 

 cular or shagreen -like surface : colour sometimes tinged with 

 yellow. 



Montagu proposed his genus Ligula chiefly to receive 

 the species which we now assign to Thracia} but, for 

 the reasons which I have given in the second volume of 

 this work (p. 433), it is inexpedient to retain that name 

 in the Mollusca. 



According to Dr. Carpenter the minute elevations or 

 points, that roughen the surface of the shell, represent 

 numerous isolated cells filled with calcareous matter, 

 and forming a superficial coating superposed upon the 

 ordinary external layer ; the epidermis is extended over 

 these points, and sinks down into their interspaces, just 

 as the human epidermis covers the papillary surface of 

 the true skin. The proper external layer is composed of 

 polygonal cells, with sharply defined boundaries, having 

 large nuclear spots strongly resembling some of those 

 which are exhibited in Mya arenaria. The total quan- 

 tity of animal matter or membrane contained in the 

 substance of the shell is extremely small, although the 

 cellular structure in all probability results from the cal- 

 cification of animal tissue. The structure of the internal 

 layer is scarcely distinguishable. The power of tension 

 * A Sea-Nymph. 



c 5 



