GASTEROPODA. 27 



a round base. The wliorls are convex ; the aperture is large 

 and rounded ; and the operculum is calcareous. A great 

 number of fossil species of this genus 

 have been described, commencing in the 

 Lower Silurian ; but there is consider- 

 able doubt as to the true position of 

 many of the older forms. 



In the genus Troclms (fig. 414, a) 

 the shell is pyramidal, with a nearly 

 flat base ; the aperture is oblique and 

 rhombic in shape, and the operculum 

 is horny. A great number of species ^'S" '''• D^outn."''''"''''' 

 of this genus, also, have been described, 



commencing in the Silurian rocks. As in the case of Turho, 

 however, the affinities of many of the older forms are very 

 problematical. 



Phasianella (fig. 414, b) is in many respects like Turho, 

 but the shell is elongated, with an oval aperture, and a smooth 

 and polished surface. Its precise range in past time is un- 

 certain, but undoubted examples occur in the Jurassic and 

 Cretaceous rocks. Monodonta in its general characters re- 

 sembles Trochus, but the columella is thickened, toothed, and 

 crenulated. The genus ranges from the Tertiary period to 

 the present day, and doubtful representatives of it have been 

 indicated as occurring in older times. In Ddijliinida the 

 shell is orbicular and depressed, the whorls angulated or 

 coronated, often spiny, the mouth round, the peristome entire, 

 and the umbilicus open. The genus seems to begin in the 

 Trias. Lastly, Stomcdclla, commencing in the Secondary 

 period, has an ear-shaped shell, with a small spire, and a 

 very wide oblique aperture. 



Fam. 16. Haliotid^. — Shell spiral, ear -shaped, or 

 trochoid ; aperture large, nacreous. Outer lip notched or 

 perforated. No operculum. Mantle - margin with a pos- 

 terior fold or siphon, occupying the slit or perforation in 

 the shell. 



The living genera Hcdiotis and Scissiirclla are not known 

 in rocks older than the Miocene Tertiary. The extinct genera 

 Pleurotomaria and Murchisonia are, on the other hand, of 



