50 THE 4ZOOLOGIST. 
of man can modify to any extent these natural conditions, 
especially in connection with their inroads. On the other hand, 
the deposits going on in the bed of the sea, the drifting of 
ordinary and coral sand, and the increase of the deltas of rivers 
are examples of agencies which tend to counterbalance the 
effects of such erosion. 
A closely allied species, Xylophaga dorsalis, which occurs 
abundantly in the deep water off the Forth (e. g. east of the Island 
of May, and off St. Abb’s Head), as well as elsewhere both in 
England and Scotland, confines its boring action entirely to 
wood. This little bivalve mollusk has two helmet-shaped convex 
valves crossed by very fine strie. Two small dorsal shields or 
plates are likewise present. It shows certain intermediate 
characters between the stone-borers just mentioned (Pholades) 
and the typical wood-borers (Teredines), having the short siphons 
of the former, the habit of perforating wood only to the extent 
necessary for its safety, and in having no calcareous lining to its 
tunnel; while in the structure of its valves it mainly leans 
towards Teredo. The late Dr. Gwyn Jeffreys, the able and 
experienced author of ‘ British Mollusca,’ is doubtful whether 
the soft parts of the animal can be contained in the shell, but in 
all the living examples both from the eastern shores and those 
from the Outer Hebrides, such can readily be retracted within 
the outline of the valves. 
In wood attacked by Xylophaga there is very little externally 
to attract attention, except the presence on the surface of minute 
apertures, which probably indicate the points by which the 
young animals obtained entrance. On breaking the wood the 
adults are found in smooth tunnels in every fragment large 
enough to afford a lodgment. Thus even a willow-basket will 
be invaded by them, especially the ribs, which are somewhat 
thicker than the other parts. The burrows are quite smooth, 
and generally contain greyish pulpy débris of minute grains of 
wood, which have passed through the alimentary canal and been 
ejected by the excurrent siphon. Most of the perforations are 
against the grain of the wood. 
The effects of Xylophaga in connection with the destruction 
of wood in British waters are considerable, especially in the case 
of submerged branches, but they fall far short, both in British 
