168 ARCID^E. 



fry have a square shape, and are only sculptured by con- 

 centric strise. Their inside margin is quite plain, and 

 has no appearance of the crenulations which are deve- 

 loped in a subsequent stage of growth. The impressions 

 left by the ligament on the triangular space between the 

 beaks in full-grown shells are very distinct. I do not 

 find that the remarkable structure of the ligament, 

 which is evident from these impressions, has ever been 

 noticed. The hinge-process is liable to become abnormal 

 or monstrous. A curious instance of it was exhibited 

 in a specimen found by Mr. Bariee in the Shetlands. 

 The hinge-plate had none of the ordinary cardinal teeth ; 

 but, by way of substitute, each valve was provided with 

 a rather strong laminar and horizontal tooth on each 

 side, which locked into a corresponding groove in the 

 opposite valve. The teeth occasionally decay and be- 

 come carious in living specimens. Whether the animal 

 suffers from tooth-ache would be a novel subject for dis- 

 cussion by the Odontological Society. Aged individuals 

 are often almost toothless, in consequence of the liga- 

 ment pushing so far forward on the hinge-plate, as to 

 obliterate all the central teeth : it reminds one of the 

 hardened gums of an old man who has lost the greater 

 part of this extremely useful apparatus. The shells are 

 often seen in grotto- work; and Mr. M' Andrew says 

 that at Algarve on the coast of Spain they are used, 

 instead of lead, by the fishermen for sinking their lines. 

 The anterior side of the shell, while the animal is alive, 

 is frequently fringed with the tubes of a Hydroid polype 

 (one of the Tubulariida) , which seems to take advantage 

 of the strong gyratory current produced by the mollusk 

 for its own food-seeking purpose. This may be an 

 analogous case to the supposed parasitic nature of Mon- 

 tacuta substriata, which is always found attached to the 



