INTRODUCTION. xiii 



ried forward in its true position by the animal, the terms right and 

 left are to be applied as they would be to any other animal. 



In addition to the usual measurements, I have adopted another, 

 for the univalve shells, which I call " divergence.' 7 It was first 

 employed by Professor Adams, under the name of u apicial an- 

 gle," that is, the angle which would embrace the spire lengthwise ; 

 in other words, it measures the tapering of the spire. 



The extent and difficulty of this work have very far exceeded 

 my expectations. The unsettled state of our nomenclature, 

 the scattered state of the materials of which it must be construct- 

 ed, have raised almost interminable- doubts and difficulties. It is 

 the first work of the kind attempted in this country ; and it were 

 presumptuous to hope that it is free from error. It is not a diffi- 

 cult thing to settle, satisfactorily, the proper genera and species 

 of nine tenths of the shells and other objects we may find. But 

 when an attempt is made to give the whole, the other tithe will re- 

 quire an equal amount of study, and, after disposing of it in the 

 best way we can, we must leave it, mortified that we have per- 

 haps settled nothing, but have merely given an opinion. It is easy 

 enough to see errors and difficulties in these cases, but it is not so 

 easy to adjust them. 



Corrections and remarks relating to the facts or opinions given 

 in the Report are respectfully solicited. 



