FAMILY 6. LITHOPHAGA. 73 



PETRICOLJ, Lamarck. 



Testa aequivalvis, insequilateralis, transversa, subtrigona vel oblonga ; 

 latere postico rotundato, antico plus minusve attenuato, plerumque 

 subhiante. Cardo dentibus duobus in utraque valva, vel acutis, 

 recurvis, vel brevibus, obtusis. Ligamentum externum. Impressio 

 muscularis pallii sinu magno. 



This is another genus in which the shell assumes a considerable irre- 

 gularity of growth from its habit of terebrating ; it may, however, be 

 readily distinguished from that of Saxicava ; the teeth are more fully 

 developed, and there is a large sinus in the palleal impression. 



owing to currents, the greatest force would be exerted at the aperture, which would, conse- 

 quently, be the largest part. At Lyme Regis, the interior of the perforation made by Pho- 

 lades was marked by parallel circular striae, owing to the mechanical action of the shell, which 

 rasps away the rock, and necessarily increases the size of the cavity in exact proportion with 

 its growth. In the other case of perforation attributed to land-snails, the action was entirely 

 chemical, and their agency was only adduced to account for appearances which the Pholades 

 could not possibly have occasioned. 



" Prof. Henslow mentioned perforations in limestone rocks from Barnstaple, in the Cam- 

 bridge Museum, which were hollowed by the chemical action of muriate of lime having 

 converted some parts into carbonate of soda. 



" Mr. R. A. C. Austen considered the borings of shells were not to be accounted for by the 

 action of acid, as the operations of the Saxicava rugosa were not confined to limestone rocks. 

 In Tor Bay the trap rocks are also perforated, and the old red sandstone is frequently found 

 bored by Pholades. He also objected to the other cavities being formed by land-snails, there 

 being but one time of the year when they remain fixed to a particular spot, and then a film 

 (epiphragm) is formed over the mouth of the shell, which is afterwards left attached to the 

 rock or other object, rendering it improbable that another snail would adhere to the same 

 place. 



" Dr. Buckland, in reply to a question by Mr. H. E. Strickland, mentioned that the locality 

 of the perforations by snails at Boulogne was six miles inland ; at Alston Moor and at other 

 places it was also far from the sea." 



VOL. I. L 



