INTRODUCTORY REMARKS. 



If, in general, it is a hazardous undertaking to describe pala30zoie fossils, whicli, 

 as a rule, have only a few recent congeners, with which to be coinpared, it is tlie 

 more so in regard to the fossil Gastropoda. Every zoologist is no doubt fnlly aware 

 of the great difiiculties, that surround his investigations, whenever he endeavours to 

 make out the aflinities and systeinatic place of fossil Gastropoda in general, and espe- 

 cially so of those of the pala30zoic era. There is indeed not much in the often distor- 

 ted shells to guide him. The microscopic structure — if there ever has been anything 

 characteristic in it, which so seldom is the case even in the recent ones — is almost 

 invariiibly destroyed or changed into a homogenous crystalline, calcareoiis spar, nor 

 are there usually any traces left of the muscular impressions, which are peculiar to 

 some few fauiilies. Moreover, what the naturalist in his study of the fossil shells has 

 to grapple with, is the same as if he had to describe recent shells only from bleached 

 and worn specimens cast ashore along the borders of the sea. There is indeed often 

 evidence that several of the Silurian fossils of Gotland and consequently also the sLells, 

 had been rolled and tossed about in the sea before being embedded in the strata, 

 which, according to all appearance, are nothing but beaches or deposits in shallow water. 

 Moreover, when once fossilized they have of necessity shared in the vicissitudes, which 

 afterwards befell the rock through metamorphic agencies, being changed as to struc- 

 ture, often deformed through pressure as to their exteriör shape, worn and eroded, 

 and consequently deprived of the fine ornamentation of the . surface. 



In almost every instance, the investigator has thus only the external shape of the 

 shell, as it is, whereupon to found his conclusions, and we know how very little the 

 empty shell informs us regarding the real nature of the animals, and how delusive, 

 even amongst recent forms, the exteriör shape can be. It may here suffice to cite 

 an instance lately brought forward by J. CarriÉre. 



In a paper on »Marginella glabella L. und die Pseudomarginellen»^) he relätes 

 his observations on two species of Gastropoda belonging, according to the structure of 



1) »Zeitschrift fiii- wissenschaftliche Zoologie Bd 37, 1882, p. 99. 



