INTRODUCTION. xi 



No alteration lias been thought necessary in the generally accepted terms of 

 longitudinal and transverse, applying them to what is so in appearance only ; and the 

 term labrum is applied to that portion of the peritreme which corresponds with the 

 right hand of the investigator ; while labium is given to its opposite or columella lip. 



For the dates of publication of the different genera, I am much indebted to the 

 work of M. A. N. Hermansen, entitled ' Indicis Genera Malacozoorum Primordia,' 

 now in course of publication ; and also to a very valuable Paper on a similar subject, 

 by J. E. Gray, Esq., published in the ' Proceedings of the Zoological Society,' 

 November, 1847. 



Considerable difficulty has been experienced in selecting an Order of Arrangement 

 for the shells described. An immense mass of information regarding the soft parts of 

 Mollusca has been obtained within the last few years ; and the animal inhabitants of 

 no less than five thousand species are now known, and have been examined ;* yet, 

 notwithstanding this additional information, we are still without a Natural Classifica- 

 tion, to which anything like a general concurrence appears to be given ; and it is only 

 necessary to compare such as have been most recently published on the Continent 

 with those of our own naturalists in England, to observe the great dissimilarity 

 between them. It has therefore been thought most advisable to employ the old arti- 

 ficial arrangement, with some slight modifications, uniting in different sections those 

 shells in which the aperture is emarginate, or furnished with a canal for the siphon 

 of the animal, and those in which the mouth is circular, or with a continuous peri- 

 treme, and merely placing in advance the Pulmonata, or air-breathing animals, after 

 the manner of Cuvier, conceiving them to be more highly organized than those which 

 breathe simply by means of gills. 



If at first sight it should be considered improbable that some of the extreme 

 varieties in form belonged to one species, it must be stated that these results have 

 been arrived at, not only by the possession, but by the careful examination of a large 

 series of individuals. 



Some information has been introduced that may be thought unnecessary ; but 

 I have considered that the newly-established Paheontographical Society will, in all 

 probability, contain among its members students just entering the field of Palason- 



* Gray, Zoological Proceedings, p. 132, November, 1847. 



