126 PHOLADIDiE. 



the form lamellata are all present in that termed 'paj)yracea^ 

 as we have convinced ourselves by an examination of pre- 

 served specimens, and as Mr. Clai-k had previously observed 

 and drawn from life in each variety. The account of 

 them in the manuscripts of that excellent malacologist, 

 communicated by Mr. JeiFreys, is so full and clear, and 

 his opinion in consequence so important on this disputed 

 point, that we cannot do better than give it in his own 

 words, though at the risk of some repetition : — " This 

 animal, in consequence of its shell having been taken under 

 very different appearances of form, has, when from circum- 

 stances it has not formed the testaceous membrane that 

 often covers its anterior ventral gape and the testaceous 

 cup-like process at the posterior end, been considered a dis- 

 tinct species, and named by Dr. Turton and others the 

 Pliolas lamellata ; but when it had acquired those appen- 

 dages, it was named PJiolas papyracea. But having this 

 summer (1835) studied the animal under both forms, we 

 are fully enabled to confirm Mr. G. B. Sowerby's opinion, 

 stated in his ' Genera,' that the two shells are one species 

 under different forms. We do not, however, think that 

 the form styled P. lamellata is the young of the shell 

 styled P. pafyracea^ but that they each maintain their 

 respective forms when of all sizes, from circumstances de- 

 pendent on peculiarities of animal economy. In corrobora- 

 tion of this opinion, we can state that we have seen what 

 is called the P. lamellata equal in size to the largest papy- 

 racea^ and, on the other hand, we have seen what is called 

 the P. papyracea completely formed, with the cup and tes- 

 taceous ventral membrane not more than a quarter of an 

 inch in length ; and we believe that when peculiar circum- 

 stances, most probably attendant on habitat and animal 

 economy arise, the animal has then the power of forming 



