SERPULA. 



imbedded in it : there are also some which^ after forming 

 a regular discoidal spire at firsts continue their shell in a 

 straight regular line. The Serpulae being irregular shells, 

 most of the variations to which they are subject, are 

 caused by peculiar circumstances depending upon the 

 substance they adhere to; thus the Spirorijes, which 

 generally are found attached to smooth and even surfaces, 

 are comparatively regular; the Vermeti also, which are 

 found imbedded in sponge, having a soft and yielding 

 substance to fix themselves to, grow freely, and produce 

 a regular pointed spire at first; but as the sponge in- 

 creases rapidly, the Vermetus ceases to form a regular 

 spire, because, in order to obtain nourishment, it is 

 compelled to keep its aperture protruded beyond the 

 sponge, and this it is only able to accomplish by an irre- 

 gular vermiform increase. 



Like the Cirripedes, the Serpulse abound in almost 

 «very situation that is at any time covered by the sea; 

 they are found attached to every thing, from the firm 

 rock and the sea weed that grows upon it, to sea animals, 

 the most rapid in their motions, such as the Lobster and 

 Sea Crayfish ; in some situations where they are not sub- 

 ject to interruption, they form patches of great thickness 

 and extent : Adanson speaks of having seen the rocks in 

 some places of the Island of Goree covered with a crust 

 several inches thick and more than twenty feet square. 



Serpula is to be distinguished from Siliquaria by the 

 longitudinal fissure of the latter, which continues the 

 whole length of the shell, but of which Serpula is desti- 

 tute : from Teredo it may be known by its tube being 

 open only at one end, and by its simple round aperture ; 

 that of Teredo being more or less distinctly double and 

 closed imperfectly by two spatulate or pennate pieces, 

 while Serpula has a regular circular operculum. 



Of fossil specie^ there are many, and they occur in 

 most strata, but little is known of them, so little that we 

 cannot distinguish any particular species as characteriz- 

 ing any strata, except one or two that appear to be pecu- 

 liar to one bed of green sand and one of Lamarck's Spi- 

 rorbes common in the chalk. The tertiary beds have 

 many very interesting and fine species. 



Our plate represents, at 



