SHELLS WITHOUT A REGULAR SPIRE. 61 



and twisted ; always affixed to other substances. Inhabits 

 the British seas. 



The Serijul£e inhabit the ocean. They generally adhere to stones, 

 shells, or plants ; sometimes a pltirality of species is found on one 

 stone or shell. There are also several species of Serpula which are 

 miattached. 



Genus 35— TEREDO. 



Animal a Terebella, with two calcareous, hemispherical 

 valves cut off before, and two lanceolate ones : shell taper- 

 ing, flexuous, and capable of penetrating wood. 



Teredo navalis — The Ship Teredo. Plate IX. fig. 35. 

 Cylindrical, taper ; smooth, white, and flexuous, finely stri- 

 ated longitudinally. 



The shells of this genus inhabit the bottoms of ships ; and poles or 

 planks, which are under water in harbours and docks. One species 

 has been found in the mud, at the bottom of the ocean. This last 

 mentioned shell is the Teredo gigantea, and has been described to 

 consist of a tube, five feet four inches in length, nine inches in circum- 

 ference at the larger, and two and one half inches at the smaller end. 

 It is the longest of all testaceous shells. 



The Teredo navalis, or ship- worm, insinuates itself into the bottoms 

 of ships, even although the oak is perfectly sound, and in a very short 

 time perforates or even completely destroys it. This destructive crea- 

 ture was, it is said, originally brought by our vessels from tropical 

 climates ; but it has now become an inhabitant of most of the harbours 

 of this island, and is very common in Plymouth Dock. 



It was first proved by that learned and very indefatigable naturalist, 

 Colonel George Montagu, in his supplement to Testacea Britannica, 

 that the animal inhabiting the Teredo, was not a Terebella, but an 

 Ascidia. More recently, however, it has been found to be neither. 



Genus 36 SABELLA. 



Animal a Nereis, with a ringent mouth, and two thicker 

 tentacula behind the head; shell tubular, composed of par- 

 ticles of sand, broken shells, and vegetable substances, united 

 to a membrane by a glutinous cement. 



Sabella tubiformis Plate IX. fig. 36. 



These shells (if they can be called so) inhabit the sea. The Linnasan 

 Sabellae are not testaceous shells, but merely coverings made up of 

 extraneous substances ; and may therefore fairly be expunged from 

 the collection of the Conchologist. They are included in Lamarck's 

 class Annelides, the formation of the animal being different from that 

 of the true MoUusca. 



