172 BIVALVES. 



only for the brilliancy of colouring, which most 

 of them display, and for the singularity of their 

 external form, but also for the clearness and 

 precision of the characters which distinguish 

 them from the shells of other genera. The 

 roughness and irregularity of their appearance, 

 their two strong incurved teeth, with the inter- 

 mediate sinus for the ligament, are their discri- 

 minative generic marks. The genus contains 

 only three species, one of which has not the 

 flattened beak and internal ligament, and its 

 shells are remarkable for being plaited longi- 

 tudinally like a fan half opened ; they have been 

 separated by modern conchologists from the 

 Spondylus under the significant name of 

 Plicatula.* The Spondyli adhere to rocks, co- 

 rals, and other marine substances, at considerable 

 depths in the sea, they are separated from them 

 with very great difficulty, they are found often 

 in groups forming large masses. The animal 

 has the edge of its mantle fringed with short 

 tentacular filaments ; it has a small radiated 

 foot. From a supposed resemblance of the 

 Spondylus to the oyster it has been called the 

 thorny Oyster ; its scientific name is derived 

 from the Greek <nrov$vXos (spondulos) the head 

 of an artichoke. 



* Plicatula, a little fold, from plica a fold. Plate VIII. 

 figure 10. 



