GASTEROPODA. 93 



The bonnet -limpets (Calyptrceidce) are common in the 

 old rocks, which also contain a few species of Chiton and 

 shells like Dentalium. The Dentalium 'primarium is from 

 Devonian limestone of Illinois and D. obsoletum from the 

 coal measures of the same part of North America. One 

 common feature of the palaeozoic spiral shells is their 

 tendency to become irregular towards the conclusion of their 

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

 all disunited; in Scoliostoma (fig. 26, 3) and Catantostoma the 

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

 ened peristome (Crossostoma, fig. 27, 5), like the recent Lietia, 

 which commences in the older tertiary. A large proportion 

 of the trochiform fossil shells have their whorls, whether round 

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

 a deep slit at the aperture ; most of these were solid nacreous 

 shells belonging to the genus Pleurotomaria, of which but a 

 single species survives; others in their slenderness resemble 

 Turritellce, and have been named Murchisonia (fig. 26, 5). 

 The Pleurotomaria sphcerulata of the Missouri coal-formations 

 has the aperture substromboidal and entire. The carbonife- 

 rous shell called Polytremaria has a row of holes in place of a 

 slit; and the Silurian Tuoina (fig. 26, 8) has three rows of 

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

 shell with one row of similar ornaments ; and Trochotoma 

 (fig. 27, 6) has a perforation near the margin of the aperture, 

 which is carried onward as the shell grows. Scissurella, 

 which is always diminutive and not pearly, makes its first 

 appearance only in the newer tertiary. Haliotis occurs in 

 the miocene of Malta. The Neritidce appear in the oolites ; 

 besides true Nerites, there are Neritomce (fig. 27, 7), with a 

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

 above (fig. 27, 8) ; and Neritopsis, with its angular columellar 

 notch most distinctly marked. Key-hole limpets (Fissurelliclce) 

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



