78 PALEONTOLOGY 



his, is probably very different ; it has a thick shelly operculum, 

 sinistral!? spiral, and furnished with an internal process, as 

 the Nerites are ; the spire is sunk and concealed, whilst the 

 whirls are exposed on the flattened under-side ; it occurs in 

 the older Silurian rocks of Scotland and North America. One 

 common feature of the palaeozoic spiral shells is their ten- 

 dency to become irregular towards the conclusion of their 

 growth : in Serpularia (= Phanerotinus, Sby.), the whirls are 

 all disunited ; in Scoliostoma (fig. 17, 3) and Catantostoma the 

 aperture is expanded. Some small oolitic shells have a thick- 

 ened peristome (Crossostoma, fig. 18, 5), like the recent Lid in, 

 which commences in the older tertiary. A large proportion 

 of the trochifurm fossil shells have their whirls, whether round 

 or angular, marked by a peculiar band, terminating in a deep 

 slit at the aperture ; most of these were solid nacreous shells 

 belonging to the genus Pleurotomariu, of which but a single 

 species survives ; others in their slenderness resemble Tarri- 

 tdlce, and have been named Murchisonia (fig. 17, 5). The car- 

 boniferous shell called Pohjtrcuwria has a row of holes in 

 place of a slit ; and the Silurian Tuoina (fig. 17, 8) has three 

 rows of tubular spines. The Cirrus of the inferior oolite is a 

 reversed shell with one row of similar ornaments ; and Tro- 

 ehotoma (fig. 18, 6) has a perforation near the margin of the 

 aperture, which is carried onward as the shell grows. Sdssur- 

 ella, which is always diminutive and not pearly, makes its 

 first appearance only in the newer tertiary. Haliotis occurs in 

 the miocenc of Malta. The Weritidce appeal in the oolites ; 

 Ik -sides true Nerites, there are Neritomcc (fig. 18, 7), with a 

 channeled outer lip ; Pilcolus, which is perfectly limpet-like 

 above (fig. 18, s) ; and Neritopsis, with its angular columellar 

 notch most distinctly marked. Key-hole limpets {Fissurcll ida) 

 occur as early as the carboniferous period, but are very scarce 

 at first, and never become numerous. The oolitic Rirn/ida is 

 a minute shell supposed to be related to a very rare living 



