256 



MOLLUSCA. 



round base. The whorls 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 considerable 

 doubt as to the true position of many 

 of the older forms. 



In the genus Trochus 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 of this genus, 

 also, have been described, commenc- 

 ing in the Silurian Rocks. As in the 

 case of Turbo, however, the affinities 



of many of the older forms are very problematical. 



The genus Euomphalus (fig. 221)15 entirely extinct, and is 



essentially Palaeozoic, ranging from the Silurian to the Trias, 



Fig. 220. Turbo subcostatus. 

 Devonian. 



Fig. 221. Euomphalus De Cewt (Billings)^ a Front view ; b View of the umbilicus. 



Devonian. 



but being most abundant in the Carboniferous Rocks. The 

 shell in this genus is depressed or discoidal, the whorls lying 

 nearly or quite in the same plane. The whorls are angulated 

 or coronated, the aperture is polygonal, the umbilicus is very 

 large, and there is a shelly operculum. The genus Ophileta is 

 closely allied to Eiiomphalus, if not identical with it. 



FAM. 1 6. HALIOTID^E. Shell spiral, ear-shaped, or trochoid; 

 aperture large, nacreous. Outer lip notched or perforated. 

 No operculum. Mantle-margin with a posterior fold or siphon, 

 occupying the slit or perforation in the shell. 



The living genera Haliotis and Scissurella are not known in 



