76 



DIVISIONS OF THE GASTROPODA. 



Fig. 670. — A, Eunema strigillahtm, 

 Ordovician (Trenton Limestone), Canada ; 

 b, Cyclonema bilix, Ordovician (Cincin- 

 nati Group), North America. (After 



Zittel.) 



rugosum, and O. sculptiim, have been described as species of Euom- 

 phalus. 



In Cyclonema (fig. 670, b) the shell is turbinated and imperforate, and 



the surface is adorned with raised 

 spiral lines intersected by finer trans- 

 verse striae. The genus ranges from 

 the Ordovician to the Devonian. The 

 Devonian genus Iso?iema resembles 

 Cyclonema, but the whorls are angular 

 and the aperture is rhombic. In 

 Eunema (fig. 670, a), again, the spire 

 is elevated, the whorls are more or 

 less angular, and the surface is adorn- 

 ed with elevated spiral ribs. The 

 genus begins in the Ordovician, and 

 the typical forms are Palaeozoic. The 

 Secondary genus Amberleya, however, 

 seems to differ from Etinema princi- 

 pally, if not solely, in the fact that the spiral ribs are replaced or accom- 

 panied by rows of tubercles or nodosities. The Silurian and Devonian 

 genus Ti'ocho?iema resembles Cyclonema, but there is a wide umbilicus, 

 bordered by an elevated ridge. In the Silurian genus Craspedostoma 

 the shell is globular, the surface has transverse lamellar ribs, and there 

 is a circular mouth " enclosed within an enormously enlarged and thick- 

 ened border" (Lindstrom). 



The genus Phasiaiiella (fig. 671) represents another type of the 



Turbinidce, in which the shell is not nacreous, and its surface is 



smooth and polished. The spire is long, the body-whorl is large, 



and the aperture is oval. The genus ranges from 



the Devonian to the present day. 



Family 13. Xenophorid;e. — This family includes 

 only the genus Xenophora (Phorus), in which the 

 shell (fig. 672) is trochoid, with a concave or flattened 

 base and keeled whorls, but differs from that of the 

 Trochidce in not being nacreous. Very commonly, 

 foreign bodies, such as shells or small pieces of 

 stone, are attached to the surface and margins of 

 the shell. The genus ranges from the Devonian to 

 the present day. 



Family 14. NerltiDjE. — In this family the shell 

 is thick and globular, with a very small spire, and 

 without an umbilicus. The aperture is semilunate, its columellar 

 side thickened and expanded, and often toothed (fig. 673), and the 

 outer lip acute. There is a calcareous, sub-spiral operculum, pro- 

 vided with a process on its inner side. The " Nerites " are in- 

 habitants partly of salt water and partly of fresh or brackish waters. 

 In many cases the inner turns of the spire become absorbed in 

 process of growth, so that casts of the interior of the fossil forms 

 often show no traces of a spire. 



Fig. 671.— Phasi- 

 anella mclanoides, 

 Eocene Tertiary. 



