116 MOLLUSCA . 



sentative, has been separated. The external tube in this 

 genus is closed at the posterior extremity, while in teredo 

 it is open. The S. polythalamia forms, according to La- 

 mark, the genus SEPT ARIA, and the two genera XYLOPH A - 

 GA and CLAVAGELLA have more recently been instituted. 

 35. SABELLA. This last genus of the Linnaean vermes 

 testacea has been degraded from its rank in conchology. 

 The covering consists of agglutinated particles of sand and 

 fragments of shells, and bears no resemblance to the testa- 

 ceous coverings of the true mollusca. It is now placed in 

 company with the terebella, and the three preceding Lin- 

 naean genera among the ANNELIDES. 



In the preceding brief review of the Linnaean genera of 

 shells, the reader will probably have been astonished at 

 those changes which have taken place. In this country we 

 are so much accustomed to the artificial method both in 

 zoology and botany, that we often reject, without sufficient 

 consideration, the improvements which the study of the natu- 

 ral method has suggested. In the time of Linnaeus, per- 

 haps, the genera of the shells, with a few exceptions, were 

 sufficiently numerous and commodious to embrace all the 

 known species ; but since the science has been cultivated 

 with more zeal, in consequence, we must say, of the intro- 

 duction of the natural method, the number of species has 

 increased tenfold. New genera and orders, and other 

 conventional divisions, have been formed, suited to the 

 state of improvement of the science. The merit of all 

 these improvements did not originate with Bruguiere or 

 Lamark, whose names we have so often had occasion to 

 mention. Many of the modern genera may be traced to the 

 systems which prevailed before the days of Linnaeus ; sys- 



