GASTEROPODA. 251 



and Tertiary formations. The sub-genus Naticopsis is Carbo- 

 niferous, Naticella is Triassic, and Globulus is found in the 

 Eocene. 



FAM. 8. PYRAMIDELLID^E. Shell turreted, with a small 

 aperture ; sometimes with one or more prominent plaits on the 

 columella. Operculum horny and imbricated. The Pyrami- 

 dellidcz commence in the Lower Silurian Rocks, and appear to 

 be on the decline at the present day. The chief fossil forms 

 belong to the genera Chemnitzia, JEulima, Loxonema, and 

 Macrocheilus. 



Gkemnitzia includes a number of slender, turreted, many- 

 whorled shells, with plaited whorls, and a simple aperture. The 

 genus appears to commence in the Permian Rocks, and whilst 

 more than one hundred and fifty fossil species are known, the 

 number of living forms is very small. Many of the shells, how- 

 ever, included under this head, are of very doubtful affinities. 



Fig. 213. Natica clausa. Fig. 214. Macrocheilus sub- 



Post- Pliocene. costatus. Devonian. 



Eulima includes small, polished, elongated shells, with level 

 whorls and a reflected inner lip. Eulima are of doubtful oc- 

 currence in the Carboniferous Rocks, are sparingly represented 

 in the Secondary Rocks, but are tolerably abundant in the 

 Tertiaries. 



The genera Loxonema and Macrocheilus (fig. 214), lastly, 

 include Palaeozoic shells, whose true place is in many cases 

 uncertain. The former extends from the Lower Silurian to 

 the Trias, but the latter is mainly, if not exclusively, confined 

 to the Devonian and Carboniferous Rocks. 



