116 MOLLUSCA. 
sentative, has been separated. The external tube in this’ 
genus is closed at the posterior extremity, while in teredo 
itis open. The S. polythalamia forms, according to La- 
mark, the genus S—pTARIA, and the two genera XYLOPHA- 
GA and CLAvAGELLA have more recently been instituted. 
35. SABELLA. This last genus of the Linnzean 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- 
nzean genera among the ANNELIDEs. 
In the preceding brief review of the Linnean 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 Linnzus, 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 Bruguiére 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 Linnzeus; sys- 
