100 PHOLADID^. 



to inhabit ? Even where it has to erode the solid rock, 

 the quantity of water it takes in, and with which all its 

 tissues are saturated, cannot fail to render the process 

 more easy. On the importance of this latter agent Valen- 

 ciennes lays considerable stress in advocating the theory 

 that the rock is worn away by the continual friction of 

 the foot. The work of perforating gneiss, in which M. 

 Cailliaud discovered living Pholades, must be extremely 

 slow and gradual. It takes probably a year and a half 

 before a Pholas arrives at maturity ; by that time it has 

 made a hole 5 or 6 inches deep. One hundredth part of 

 an inch may therefore be reckoned its daily task. Time 

 is of course a necessary element in all operations ; and 

 it serves no less to advance the labours of the persevering 

 and patient shell-fish, than to scoop out valleys by the 

 agency of running waters and yielding glaciers, 



" And waste huge stones with little water-drops." 



The Pholadida are distributed over the greater part 

 of the globe ; but the species, although prolific, are not 

 numerous. According to Searles Wood " Pholades have 

 been found fossil as early as the Lias"; and Chenu 

 says that they occur in the Jurassic, cretaceous, and 

 tertiary formations. 



They comprise with the Teredinid<e the multivalves 

 of Adanson and other writers of a later date ; but neither 

 the dorsal shields possessed by some species of Pholas, 

 as well as by Pholadidea and Xylophaga, nor the sheath 

 and pallets of Teredo are " valves " in a conchological 

 sense, any more than the opercula of many univalves. 

 Nor are any of these appendages homologous, or formed 

 by similar organs. Linne also considered Pholas a 

 multivalve, and placed it with Chiton and Lepas. Pul- 

 teney was not so far from the mark when he conjectured 



