271 



of an outer, and an inner ov colionellar Up. The peristome is complete 

 when both inner and outer lip are present, and incomplete when the place 

 of the inner lip is taken by the preceding whorl. In a great many species 

 the peristome is notched anteriorly, or produced into a straight, or more 

 or less flexed, canal. A posterior notch is also frequently found. The 

 columellar lip, and in its absence the columella, may be smooth or 

 furnished with one or more plications. Similarly, the outer lip may be 

 smooth on its inner side or furnished with plications or lircp. Among the 

 external features of importance are the transverse lines of growth, which 

 mark the successive increments; varices or rows of spines, parallel to the 

 lines of growth, and marking periodic resting stages during the growth 

 of the shell; and revolving longitudinal lines or ridges, which may be 

 uniform or alternating, or may show a gradation in size. When the varix 

 is reduced to a single spine, this usually marks the angle between a 

 shoulder and the body of the whorl. The shell will thus come to be 

 ornamented by a revolving row of spines set at regular intervals on the 

 shoulder-angle. In place of the spines, simple nodes may occur. When 

 transverse and longitudinal lines cross each other, a reticulated surface 

 ornamentation is produced ; and when the shell is covered by an epider- 

 mis, or periostracum, hair-like spines not infrequently arise from the 

 points of crossing. In Pleurotomaria and related gastropods, a siphonal 

 notch occurs in the outer lip, and its progressive closure from behind 

 leaves a marked revolving band, commonly visible only on the body- 

 whorl. 



Many species, especially of marine gastropods, secrete a horny or 

 calcareous operculum, which is attached to the foot, and closes the 

 aperture of the shell when the animal is withdrawn. This is seldom pre- 

 served in fossils. 



Note. — The anatomy of gastropods is treated of in the text-books on 

 zoology. The various manuals of conchology describe the recent as 

 well as the fossil genera, with especial reference to the shells. For a 

 modern classification, Zittel's text-book (Eastman's translation) should be 

 consulted. The species described beyond are nearly all more fully de- 

 scribed and illustrated in Vol. V., Pt. II., Pal. N. Y., to which reference 

 is made in the text. 



Genus PLATYCERAS. Conrad. 



[Ety. : Platys, flat ; keras, horn. ] 

 (1840: Ann. Rep. N. Y., p. 205.) 



Shell conical, irreg-iilar, with or ^Yitho^t the apex inrolled. 

 Aperture expanded, often reflexed; peristome entire, often 

 sinuous. Surface variously striated, sometimes bearino- 

 spines. 



