300 MOLLUSCA FROM THE CRAG. 



concentric layers, thickened with the age of the animal. If 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 (tfipadites, 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 Teredo, 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 Linnaeus, 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 Teredo 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. 



TEREDO NORVAGICA, Spengler. Tab. XXX, 12, a d. 



TEREDO NORYAGICUS. Spengler. Skrivt. af Naturh. selsk., vol. ii, part 1, p. 102, pi. 2, 

 figs. 46 B, 1792. 



NORVAGICA. Loven. Ind. Moll. Scand., p. 50, 1846. 



Forb. and Hani. Hist. Brit. Moll., vol. i, p. 67, pi. 1, figs. 1 5, 

 1848. 



NORVEGICA. 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. 



