128 



Rocks in Cumberland and Westmorland. In this deposit great 

 numbers of fossils are crowded together, as at Penton, and they 

 include quite as great a variety of forms. Owing, however, to the 

 fact that the calcareous portion of the rock has been carried away 

 in solution through weathering, the fossils in this instance are 

 represented chiefly by impressions of the exterior, together with 

 casts of the original vacancies. By carefully clearing away the 

 extraneous matter, and then pressing modelling-wax into the 

 cavities left by the shells, impressions are obtained which reproduce 

 all the original relief of the shell, even to minute details only 

 discernible under a lens of high magnifying power. 



Fossil remains of Gasteropoda rarely present any traces of 

 muscular impressions, or of such structures as in other MoUusca 

 are available as guides to the zoological position of the animals 

 that formed them. The opercula, where such originally existed, 

 are but seldom found, and the lingual dentition is in nearly every 

 case quite unknown. Hence we are compelled to rely, for the 

 determination pf generic or of specific characters, upon the data — 

 most of them unsatisfactory at the best — that are furnished by the 

 external characters alone. 



Amongst these external characters, the form of the mouth, and 

 especially the form of the outer lip, is one of the features of greatest 

 value, as indicating, to some extent, structural differences in the 

 animal that formed the shell. But a perfect mouth to a shell is 

 rare in the fossil state, and we are therefore obliged to make a 

 close examination of the structural feature that represents, in itself, 

 a succession of positions occupied by the edge of the outer lip. 

 They are represented by the lines of growth. And even these are 

 not always of such a nature as to furnish evidence of much value, 

 as individual specimens of such shells as usually display well-marked 

 lines of growth, occasionally have them obliterated. It therefore 

 becomes necessary, before stating any conclusion with regard to the 

 specific or other relations of fossil Gasteropoda, to make a careful 

 examination of a large number of specimens. 



In the case of the species about to be described, I have made a 

 .careful study of nearly the whole of the literature on the subject, 



