52 LINNEAN GENERA. 



The shells of this genus inhabit the sea, lakes, and rivers. Those of the 

 sea in general adhere to rocks and stones on the shore ; and are so tenacious, 

 that they cannot be removed without breaking the shell, unless taken by- 

 surprise. Those inhabiting fresh water are generally found attached to 

 aquatic plants. The Patellae are called Limpets in Britain. 



w Chamber. 



Genus 33.— DENT A LI UM. 



Animal a Terebella ; shell univalve, tubular, straight or slight- 

 ly curved, with the cavity undivided, and open at both ends. 



Dentalium entalis. — The Tooth Dentalium. Plate IX. 

 fig. 33. White or yellowish, slightly curved, and tapering to a 

 fine point ; slightly striated. One and a-half inch long. Inha- 

 bits the British seas. 



The shells of this genus are only found in the ocean. They are solitary, 

 and lie in the mud, in an oblique or perpendicular position. 



Genus 34.— SERF UL A. 



Animal a Terebella ; shell univalve, tubular, generally ad- 

 hering to other substances ; often separated internally by divi- 

 sions at uncertain distances. 



Serpula vermieularis — The Worm- Like Serpula. Plate 

 IX. fig. 34. White, cylindrical, tapering, and variously curved 

 and twisted ; always affixed to other substances. Inhabits the 

 British seas. 



The Serpulse inhabit the ocean. They generally adhere to stones, shells, 

 and plants ; sometimes a plurality of species are found on one stone or shell. 

 There are also several species of Serpula which are unattached. 



Genus 35 TEREDO. 



Animal a Terebella, with two calcareous, hemispherical valves 

 cut off before, and two lanceolate ones : shell tapering, flexuous, 

 and capable of penetrating wood. 



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

 Cylindrical, taper ; smooth, white, and flexuous, finely striated 

 longitudinally. 



The shells of this genus inhabit the bottoms of ships ; and oak poles, which 

 are under water in harbours and docks. One species has been found in the 

 mud, at the bottom of the ocean. Tliis 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 circumference at the larger, and two and one half 

 Liches at the smaller end, and is the longest of all testaceous shells. 



