MONTACUTA. 215 



become coated with a thick but irregular layer of hard 

 ochreous dirt, like iron-rust. I suspect that this in- 

 crustation is caused by a continual deposit and accumu- 

 lation of faecal matter from the animal, which is not 

 carried off in consequence of its sedentary habits and of 

 the water in which it lives being free from currents. 

 The same remark applies to species of Pisidium and 

 other freshwater shells, as well as to many of the ma- 

 rine bivalves. Forbes and Hanley observe that M.fer- 

 ruginosa is " a scarce shell." This of course is only 

 in comparison with other species; but the common 

 notion of rarity is not quite correct. I consider that 

 " local" would be a more appropriate word than "scarce" 

 with regard to the occurrence of marine testacea. The 

 difficulty of procuring some particular kinds may en- 

 hance their value to collectors ; but probably all kinds 

 are prolific, and differ in that respect from many of the 

 larger land mammals. There certainly cannot exist a 

 " unique" specimen of any shell in nature. In the 

 present case single valves may be found in tolerable 

 plenty on several of our sandy beaches, and perfect spe- 

 cimens may be got by dredging in the sheltered lochs of 

 the west of Scotland and in rather deep water off our 

 southern coasts. The beaks are often fissured or notched 

 from their being squeezed close together by the strong 

 and elastic cartilage, the shell being too fragile to resist 

 such pressure. 



The proportions of length and breadth vary greatly 

 in specimens, which may account for the diversity of 

 names which this species has from time to time re- 

 ceived. As I have already incidentally noticed, it ap- 

 pears to be the Amphidesma purpurascens of Lamarck ; 

 Brown called it Tellimya elliptica and T. glabra, 

 S. Wood T. ovata, Leach Amphidesma Goodalliana, 



