Fig. 113.—Radula of Bela turricula Mont. × 70.
Fig. 114.—Portion of radula of Spirotropis carinata Phil., Norway. × 70.
Fig. 115.—Eight teeth from the radula of Terebra caerulescens Lam. × 60.
The arrangement of teeth in all these sections is expressed by a formula applicable to each transverse row of the series. The central tooth, if present, is represented by 1, and the laterals and marginals, according to their number, on each side of the central figure. Thus the typical formula of the Toxoglossa is 1.0.0.0.1, the middle 0 standing for the central tooth which is absent, and the 0 on each side of it for the absent laterals; the 1 on each extreme represents the one uncinus in each row. Thus the formula for Spirotropis, which has also one lateral on each side and a rachidian or central tooth, is 1.1.1.1.1. Often the formula is given thus:
where 30 and 42 stand for the average number of rows of teeth in Conus and Spirotropis respectively; the same is sometimes expressed thus: 1.0.0.0.1 × 30; 1.1.1.1.1 × 42.
Fig. 116.—A tooth from the radula of Conus imperialis L., S. Pacific, × 50, showing barb and poison duct.