PROSOBRANCHIATA. 



775 



Fig. 667. — Sa's- 

 surella aspera. 

 Pliocene Tertiary. 



opercula, even when shelly and massive, are not often met with 

 in the fossil condition. 



Monodonta in its general characters resembles Troc/ius, but the 

 columella is thickened and carries a prominent tooth. 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 Deiphinula 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 Jurassic rocks. In Umbonium the shell is 

 circular, with a short spire and a large body-whorl, the 

 surface being polished, and the umbilical region covered 

 with a prominent callosity. The genus begins in the 

 Devonian, and still survives. Lastly, according to the 

 views of Lindstrom, we must include in this family the 

 genus Scissiirella (fig. 667), in which the shell is thin, 

 with a greatly expanded body-whorl, and the outer lip is 

 furnished with a deep slit. The earliest forms of Scissiirella appear in 

 the Cretaceous rocks, and the genus still survives. 



Family 12. Turbinid^e. — The shell in this family is solid and 

 nacreous internally (except in Phasianella), turbinated or trochoid 

 in form, with an entire, round or oval aper- 

 ture, and a rounded or flat base. The oper- 

 culum is calcareous, paucispiral, with a central 

 or excentric nucleus. The members of this 

 family are marine, and inhabit shallow water. 



In the genus Turbo (fig. 668) the shell is 

 turbinated, with a round base, the whorls 

 being convex, and the aperture large and 

 rounded. The species of the genus range 

 from the Silurian to the present day, about 

 four hundred fossil species having been de- 

 scribed ; but owing to the non-preservation 

 of the opercula there is much doubt as to the true position of 

 many of the older forms. 



In Astralium (Imperator) the shell is trochoid, with a flat or concave 

 base, and with keeled or foliaceous whorls and an angulated aperture. 

 The genus ranges from the Trias to 

 the present day. The genus Orios- 

 toma (fig. 669) comprises Silurian and 

 Devonian forms, in which the shell is 

 discoid, with a short spire, the whorls 

 being in contact, but the last whorl 

 sometimes partially disjunct. The 

 surface is usually spirally ribbed ; 

 there is a wide and open umbilicus ; 

 and the aperture has thin lips, with- 

 out a sinus. The inner layer of the 



shell is nacreous, and there is a solid calcareous operculum. Some of 

 the most familiar of the Silurian forms, such as O. discors (fig. 669), O. 



-Turbo subcostatus. 

 Devonian. 



Fig. 669. — Oriostoma (Euomphalus) 

 discors. Silurian, Britain. 



