INTRODUCTION. 



XI 



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 Classification, 

 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 

 artificial 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 

 peritreme, 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 Palseontographical Society will, in all 



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



/ 



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



