584 NORTH AMERICAN INDEX FOSSILS. 



(Fissurided), or by periodic renewal as a succession of holes 

 (Trematonotiis). The outer emargination is often only indicated 

 by the reflected course of the lines of growth on the shell. On 

 the inside of the outer lip various ridges or plications called lirce 

 are sometimes found, and these occasionally may be strong and 

 tooth-like (Nerined). Similar ridges or colnmellar pliccz or folds 

 are more often found on the inner lip, next to the columella or 

 central spiral twist. These may be oblique or normal to the 

 axis of coiling (horizontal), few or numerous, readily seen, or far 

 within the shell so as to be invisible except in broken shells. 

 When the axis of coiling is hollow (perforate spire) the opening at 

 the base constitutes the umbilicus. This varies greatly in size, 

 and may be wholly or in part covered by an expansion or callus 

 of the inner lip {Natica)* 



Most modern shells are covered by a horny smooth or hairy 

 epidermis or periostracum, which hides the (often brilliant) color 

 markings of the surface. This, as well as the coloration, is rarely 

 preserved in fossil shells. 



The apertural end of the gastropod shell is the anterior end, 

 the apex of the spire the posterior. Most authors figure the shells 

 with the apex of the spire uppermost. French authors generally 

 figure them with the anterior end uppermost. The aperture is 

 often closed by a horny or calcareous operculum, of very variable 

 form in the different groups. It is secreted by and attached to 

 the foot of the animal. 



Literature. 

 A. Paleozoic Species. 



1847-1879. Hall, James. Palaeontology of New York. Vols. I., 

 II. and III. and Vol. V., Pt. II. 



1861. Billings, E. Palaeozoic Fossils. Vols. I. and II. 



1908. Raymond, P. E. The Gastropods of the Chazy Formation. 

 Annals of the Carnegie Museum, Vol. IV., nos. 3 and 4. 



1908. Ulrich, E. O., and Scofield. The Lower Silurian Gastro- 

 poda of Minnesota. Minn. Geol. Surv. Palaeontology, Pt. II. 

 (This is in many respects the most important work on American 

 Palaeozoic Gastropods. ) 

 For descriptions of American Palaeozoic species, see further : The 



* For further discussion of these characters see A. W. Grabau, " Studies of Gastro- 

 poda, I. and III." {Am. Nat., XXXVI., No. 432, and XLI., No. 490.) 



