Page:Land Mollusca of North America (north of Mexico) Vol. I Part 1 i-276.pdf/12

XII Colors, except in some of the quoted descriptions, are those of Ridgway's Color Standards and Nomenclature. For use with land shells, this work is not wholly satisfactory, as more nuances and dilute tints of the browns are needed. But, while particular hues are often characteristic, it must be admitted that shells frequently show a wide range of individual variation in color as well as in tone. The term "horn color" or "corneous," used by some authors, might well be discarded. It dates back to the time when articles made of horn were in use. The translucent, grayish-white tints intended can be otherwise expressed more intelligibly. It was not used by Ridgway, and we have no standard showing just what tint is meant.

The terms used in descriptions of pallial organs and genitalia are sufficiently shown in page 3, Figures 1, 2 and page 65, Figure 31.

As to illustrations of medium or large snails, I have found natural size figures, with enlarged details, where necessary, to be more readily recognizable than the enlarged figures sometimes given, which are usually too big to look natural, and not large enough to show the details of sculpture. In this work, most halftone figures were prepared by Miss Helen Winchester, when not otherwise credited. The original line drawings of shells and anatomy are by the author.

Terminology of stages of growth.—As some of the terms for stages of growth have often been used in descriptive and other matter relating to helices, the terms originated by Alpheus Hyatt are explained here so far as applicable to gastropods.

Embryonic. From fertilization of the egg to culmination of podocyst and other specially embryonic structures. Since the shell record of the succeeding stage (nepionic) is extremely brief, the term embryonic is practically used to cover the stage ending with birth or escape from the egg capsule. The shell in the embryonic stage usually has a special sculpture, as in page 226, Figure 112, following an initial smooth or radially rippled fraction of a whorl, or it is smooth throughout, and thus more or less unlike the following whorls.

Nepionic. Decadence and disappearance of specially embryonic structures. The snail assuming the final form externally. Very short in land snails; when distinguishable in a shell it is indicated by a striate or smoothish streak, or a slight contraction, at the end of the embryonic part.

Neanic. Youth, from birth up to completion of the shell, which is usually coincident with the beginning of reproductive activity.

Ephebic. Stage of maturity. Shell and lip usually become thicker.