ODOSTOMIA. 113 



colour. The inversion of the apex was first pointed out 

 by Montagu. That this part of the spire is likewise 

 sinistral, or turns to the left hand instead of to the 

 right, has been since ascertained, and is a still more 

 anomalous fact : it is a good and constant feature of 

 the genus. Owing to the species being generally so 

 prolific and widely diffused,, it is excessively difficult to 

 define their exact limits, and to say which forms are 

 specific and which varietal. I endeavoured to perform 

 this undertaking in a monograph which was inserted 

 between eighteen and nineteen years ago in the ' Annals 

 and Magazine of Natural History/ With the aid of 

 subsequent experience and greater opportunities of com- 

 parison, I will now revise my work, professing (and 

 indeed intending, so far as human nature permits) to 

 treat my own discoveries with a share of justice not 

 less rigorous than that which I measure out to my 

 brother conchologists. I have no ambition to be a species- 

 maker, much less have I any desire to invite that appel- 

 lation. I will do my best, by descriptions and figures, 

 to help collectors in making out what I consider true 

 species. But I must at the same time confess having been 

 not seldom puzzled by intermediate forms; when I almost 

 fancied that these paradoxical lines in the ' Passionate 

 Pilgrim ' had reference to my perplexity: 



" Keason, in itself confounded, 

 Saw division grow together ; 

 To themselves yet either neither, 

 Simple were so well compounded ; 



That it cry'd, how true a twain, 

 Seemeth this concordant one." 



To show how other conchologists have failed in deter- 

 mining certain species, let me instance O.plicata, Mont. 

 Macgillivray mistook for it a worn 0. spiralis, S. Wood 



