Page:Land Mollusca of North America (north of Mexico) Vol. I Part 1 i-276.pdf/11

 Measurements, colors, etc.—Measurements are now always taken with a caliper rule, parallel to lines indicated in Figure A; but in actual practice, the greatest diameter measurable is generally used with strongly depressed helicoid or discoidal shells. The diameter strictly at right angles with the axis cannot easily be measured so exactly. In comparing modern measure- ments with those of old descriptions it must be remembered that most older authors used a flat rule, and they usually measured the height of helicoid shells along the shell axis, instead of to the edge of the basal lip of aperture. One cannot expect modern measurements of a type-specimen to agree exactly with those published fifty or a hundred years ago.

Fig. A, diagram to show method of measuring: a-b, diameter; a-e, height, B, count of whorls.

The older authors, and many to this day, gave two diameters, greater and lesser or major and minor, in measuring helicoid shells. This was a hangover from the Pfeifferian era when genera were much more hetero- geneous than at present. In shells with simple lip, such as Retinella or Mesomphix, or those with irregular coil, the minor diameter is significant, though even so, it might better be expressed as the ratio of minor to major diameter. In shells with expanded or reflected lip, the minor diameter is superfluous in practical work, as nobody uses it when given. It appears in this work only in quoted descriptions.

The proportion of height to diameter (h/d ratio) is taxonomically use- ful in a broad way, but in species of many of our genera it is subject to great individual variation.

The whorls are, of course, numbered from the apical down, as in the right-hand column in Figure B; but in counting them it appears to me easier to count from the last one up to the first, as in the left-hand numerals, Figure B. (XI)