GASTEROPODA. 



257 



rocks older than the Miocene Tertiary. The extinct genera 

 Pleurotomaria and Murchisonia are, on the other hand, of great 

 antiquity, the latter being exclusively Palaeozoic, and the 

 former mainly so. 



The genus Haliotis comprises the so-called " ear-shells," 

 distinguished by their ear-shaped shell, with a minute spire, 

 an enormous aperture, and a series of round perforations in 

 the outer angle of the shell. A few fossil species are known, 

 commencing in the Miocene. 



In the genus Srissurella, which also commences in the 

 Miocene, the shell is thin, with a large and greatly expanded 

 body-whorl, and the place of the perforations of Haliotis is 

 taken by a simple slit in the margin of the outer lip. 



The genus Plettrotomaria comprises a great number of 

 Palaeozoic univalves, which occur in the Silurian, Devonian, 

 and Carboniferous formations. In sediments later than the 

 Carboniferous the genus is largely represented, extending even 

 to the close of the Mesozoic period. In the Jurassic period 

 especially the genus has a great development, most of the 

 forms being more ornate than those from the older rocks. 

 In the Cretaceous Rocks the genus finally dies out, with 

 the sole exception of a single living species. The form of 

 the shell in Pleurotomaria (fig. 222) differs considerably in 

 different cases. Very 

 commonly the shell is 

 very similar to that of 

 Trochus. In other cases 

 it more nearlyresembles 

 Turbo ; and sometimes 

 it is very much flatten- 

 ed out and depressed. 

 The shell consists of 

 few whorls, of which 

 the last may be discon- 

 nected from the others, 

 and is essentially dis- 

 tinguished by its sub- 

 quadrate aperture, with 

 a deeper or shallower 

 slit in the outer lip. As 

 the shell grows, this slit 

 becomes progressively filled up, forming a well-marked band on 

 the whorls. By this character Pleurotomaria may generally be 

 distinguished readily from such shells as Trochus and Turbo. 



Murchisonia (fig. 224) is another genus of great importance 



R 



Fig. 222. Pleurotomaria Agave. Lower 

 Silurian (Billings). 



Fig. 223. Pleurotomaria Dryope. Lower 

 Silurian (Billings). 



