96 PHOLADIDvE. 



in Pholas there were eyes analogous to those with which 

 the scallop is furnished ; hut Blanchard could not detect 

 any such organ, although he had traced all the nerves in 

 this part of the hody to their extremities. Bora's view, 

 that the hinge is connected by a ligament, was adopted by 

 Clark. It is incorrect. Pholas has no ligament, concho- 

 logically speaking ; and its proper function, that of open- 

 ing and closing the valves, is performed by the anterior 

 adductor muscle. The shells are white or colourless, 

 owing to their confined position. Their composition is ex- 

 ceedingly firm, and partakes of the nature of arragonite. 

 This is sometimes necessary, in order to sustain the 

 almost constant pressure of the shell against hard rocks. 

 Adanson seems to have mistaken the nature of the 

 dorsal shields (or " accessory valves," as they have been 

 also called) when he used the same word, " palettes," to 

 designate these appendages and the opercular bars of 

 Teredo. The falchion-shaped shelly processes which 

 issue from the hinge were first observed by Lister, 

 and called "apophyses." Klein afterwards applied the 

 same term to the dorsal shields. The true apophyses 

 were regarded by Deshayes as cardinal teeth. But it 

 seems to me that they have nothing to do with the 

 hinge, and that they are formed by a different part of 

 the mantle. They are probably of service in keeping 

 the viscera in their proper place, and protecting them 

 from the strain caused by the muscular exertions of the 

 animal in the act of boring. 



The notion that the shell is the instrument of perfo- 

 ration originated with Bonanni, in 1684. It was adopted 

 in the last century by Adanson, Born, and others ; and 

 in the present century most zoologists of note and expe- 

 rience have favourably entertained it. No one, however, 

 can be compared to M. Cailliaud of Nantes in respect 



