300 MOLLUSCA FROM THE CRAG. 
concentric layers, thickened with the age of the animal. //the Sumatra species cor- 
respond in all other respects with this genus, it will slightly diminish the value we 
have hitherto placed upon structural composition. 
This genus is found abundantly imbedded in the fossil wood of the London Clay, 
and the fruits or seed-vessels of Sheppey (Vipadites, Bowerbank) are sometimes per- 
forated by these animals. The tubes are of various sizes, some of them much smaller 
than others, the larger ends of which are generally closed, and consequently presumed 
to have attained to the full period of individual existence. The closing of the larger 
end is an operation performed also by other genera, such as Clavagella and Aspergillum, 
which have a perforated termination, and the valves are fixed or soldered into the sides 
of the tubes. This fixing of the valves is only done when the animal constructs the 
roof to its dwelling, as, if fixed at an earlier period, they would appear at irregular 
distances ; and I believe the valves of these genera, like those of Zeredo, are loose and 
free while the animals are growing, and only imbedded in the walls of the tubes at the 
time they have closed or imprisoned themselves by the construction of their dome- 
shaped or perforated disk. 
There is no true ligament in this genus for the union of the valves, but a powerful 
muscle forms a deep impression upon the shell at the exterior of the dorsal margin, 
and is the only hinge on which the valves are moved. 
A species has been found in the Lias in dichotyledonous wood. 
The natural history of this “calamitas navium,” as it was called by Linnzus, is 
graphically given by the authors of the ‘ Hist. of Brit. Moll., with the various opinions 
respecting these wood-eating animals, and their allies, the stone-eating Pholades. This 
subject is far from being exhausted, differences of opinion as to the modus operandi 
exist at the present day, and have done so ever since the time of Pliny, who imagined 
the animal of the Zeredo to be a worm, and its two valves the formidable jaws by which 
it was capable of inflicting upon mankind such dire calamity. The species are not 
numerous, even in the recent state, but the individuals have multiplied to an alarming 
extent, and at one time threatened to submerge the States of Holland. 
Trerepvo Norvaaica, Spengler. Tab. XXX, 12, a—d. 
Trrepo Norvaaicus. Spengler. Skrivt. af Naturh. selsk., vol. ii, part 1, p. 102, pl. 2, 
figs. 4—6 B, 1792. 
— Norvaerca. Lovén. Ind. Moll. Scand., p. 50, 1846. 
-—— — Forb. and Hanl. Hist. Brit. Moll., vol. i, p. 67, pl. 1, figs. 1—5, 
1848. 
— Norvectca. Thompson. Ann. and Mag. Nat. Hist., vol. xx, pp. 157, 163. 
— — Adanson. Acad. Scien. Par., t. 9, figs. 1—8. 
— NavaLis. Mont. Test. Brit., p. 527; and Supp., p. 7. 
