268 INTRODUCTION TO CONCHOLOGY. 



beauty of the Spondylus, or Thorny Oyster. The shell of 

 this interesting genus is thicker and of more irregular 

 growth than that of Pecteiij it is uniformly attached by 

 one or both valves^ and a marked change is perceptible 

 in the hinge, which is provided with strong teeth, and a 

 solid umbonal area. 



The animal inhabitant apparently possesses abundance of 

 calcareous matter, for as the shell advances in growth, the 

 valves are composed of several distinct plates, deposited one 

 upon the other in progressive order. In the Spondylus 

 varius this laminar structure is very remarkable; the 

 plates are so irregularly secreted as to admit water, which 

 becomes stationary by the sealing up of the last plate. The 

 transparent Water Spondyls are therefore highly estimated. 



The shell of the Spo^iclylus is attached, inequivalve, 

 somewhat irregular, and more or less auriculated. The 

 outer surface is rarely smooth, and beautiful are the va- 

 rieties of external development wdiether of spines or folia- 

 tions ; the colours, also, are exceedingly bright and vivid. 



While comparing this gracefully- decorated shell with 

 others of equal vividness, whether in cabinets, or placed by 

 the waves within our reach, we shall have occasion to 

 observe that the coverings of different mollusks bear not 



