372 BIVALVES. 



only for the brilliancy of colouring, which most 

 of them display, and for the singularity of their 

 external form, hut also for the clearness and pre- 

 cision 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 intermediate sinus 

 for the ligament, are their characteristic generic 

 marks. The genus contains only three species, 

 one of which has not the flattened beak and in- 

 ternal ligament, and its shells are remarkable for 

 being plaited longitudinally like a fan half opened ; 

 they have been separated by modern concholo- 

 gists from the Spondylus under the significant 

 name of Plicatula.* The Spondyli adhere to 

 rocks, corals, 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 a-vovtivhoq (spondulos) the head 

 of an artichoke. 



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

 figure 10. 



