MoUasca of St. Andreivs. 343 



skilful mussel-farming, great increase in revenue might be 

 anticipated. Shell-fish, like other animals in civilized parts, 

 cannot survive constant inroads without special restrictions. 

 Multitudes o^ the young mollusks, moreover, are found in- 

 crusting the poles for the salmon-nets on the West Sands, and 

 the rocks, stones, and tangle-roots elsewhere ; but they do not 

 attain a large size, apparently from overcrowding and the want 

 of congenial food, which the purer water seems incapable of 

 supplying. This species takes the place of the horse-mussel 

 ("yoags '") of the Zetlandic fishermen, and the worms of those 

 in the Channel Islands. It is seldom eaten by the natives. The 

 edible cockles, again, abound on the sandy flats near the 

 entrance of the Eden into the sea, and are occasionally sold as 

 food, though of late years their scarcity has rendered their 

 appearance less frequent in the market. Periwinkles and 

 limpets are constantly gathered for similar purposes. The 

 only uses to which some of the other mollusks are applied are 

 in the amateur manufacture of ornaments, such as shell pin- 

 cushions from various univalves and bivalves, bracelets from 

 Nassa incrassata and Trochus cinerarius, after the latter has 

 had its nacreous layer exposed by an acid. 



The rock-boring shell-fish are five in number, though only 

 one exercises any great influence on the disintegration of the 

 rocks — viz. Pholas crisjxita, which often swarms in the shale 

 and sandstone, and takes the place of the PJiolas dactylus of 

 the chalk rocks of the south. Pholas Candida occurs too rarely 

 to require special mention in this respect -, and the same may 

 be said of Saxicava rugosa. The excavations of Patella vul- 

 gata and CJ ikon fascicular is in sandstone show that no special 

 boring-organ is necessary for this purpose. The latter species 

 sometimes scoops out considerable cavities in sandstone, in 

 which it reposes. The only wood-borer is Xylophaga dorsalis. 



In taking, under this head, a general survey of the boring 

 forms, it is found that they belong to at least three invertebrate 

 subkingdoms, viz. the Protozoa, MoUusca, and Annulosa. In 

 the first are boring sponges ; in the second, Bryozoa and various 

 mollusks ; in the third, sea-urchins, gephyreans, annelids, and 

 cirripedes. 



The sponges appear to bore only into calcareous substances, 

 such as shells and limestone. The Bryozoa perforate shells ; 

 the mollusks proper, limestone, sandstone, aluminous shale, 

 gneiss and other rocks, wood, wax, shells, &c. ; the annelids 

 tunnel shells and rocks of various kinds ; the sea-urchins cal- 

 careous rocks, gneiss, granite, and other rocks ; the gephyreans 

 and cirripedes shells and limestone. Good opportunities are 

 afforded at St. Andrews for studying the boring-action of the 



