774 



DIVISIONS OF THE GASTROPODA. 



Cambrian and the base of the Ordovician series (the Quebec Group 

 and Chazy Limestone in North America, the Durness Limestone in 

 Scotland, &c.) 



Finally, we may notice here the Devonian fossil for which Billings 

 proposed the name of Euomphalus De-Cewi, and upon which Hall 

 has based the genus Pleuronotus. In this form (fig. 664) the shell 

 is discoidal, with a wide shallow umbilicus and a concave spire, the 



outer lip of the aperture showing a dis- 

 tinct sinus. The surface on the sum- 

 mit and external side of the whorls is 

 "marked by a distinct band, to the 

 margins of which the surface-striae con- 

 verge on the two sides, and upon which 

 they make a regular retral curve " (Hall). 

 This last character renders it probable, 

 as suggested by Hall, that Pleuronotus 

 should be referred to the family of the 

 PleurotoinariidcE. 



Family 10. Solariid^:. — In this 

 family the shell is obtusely conical or 

 depressed, with a large and deep um- 

 bilicus running from the base to the 

 apex of the shell. There is no na- 

 creous layer, and the aperture is entire. 

 The principal genus in this family is Solarium itself (fig. 665), 

 comprising the familiar " Staircase Shells." In this genus the 

 edge of the umbilicus is typically crenulated, and the aperture is 

 rhombic. The species of Solarium range from the Cretaceous 

 (Trias ?) to the present day. 



Family ii. Trochid^e. — In this family the shell is conical, 

 pyramidal, top-shaped, or helicoid, the aperture being entire, and 

 either quadrilateral or round in form. The shell 

 is nacreous, and the operculum is horny, cir- 

 cular, and multispiral, with a central nucleus. 

 The members of this family are all marine, and 

 are mostly found between tide-marks or in shal- 

 low water; a few forms, however, being inhabi- 

 tants of deep water. 



In the genus Trochus (fig. 666) the shell is 

 pyramidal, with a nearly flat base, and the aper- 

 ture is oblique and rounded in shape. The range 

 of the genus is from the Silurian to the present 

 day, but the determination of many of the fossil forms is un- 

 certain, since it is by the structure of the operculum that the 

 genus is essentially distinguished from Turbo and Astralium, and 



Fig. 665. — Solarium ornatum. 

 Gault (Upper Cretaceous). 



Fig. 666. — Trochus 

 conulus. Pliocene Ter- 

 tiary. 



