An inspector would need to be equipped with an ordinary rectangular caliper. If a shell should be found to measure more than 2 inches in any linear direction it would be considered as above the size limit.
CLOSED REGIONS--NECESSITY AND APPLICATION.
In addition to the provision of size limits it is strongly recommended that certain portions of the rivers be closed for rest periods covering several years. It might be thought that in regions of extreme depletion the operation of a size limit would, by making the fishery less profitable, have the effect of causing a practical rest period, but this can not be expected, for, stimulated by the high price of shells and the ever-present hope of making a pearl find, the local shellers will hardly ever desist entirely from the fishery.
No better way of giving protection to mussels can be found than that of entirely stopping the shelling upon a series of beds, although the plan must be applied in such a way as not to reduce the supply of mussels unduly and suddenly and with as careful regard as possible to the established interest of communities.
Some of the conditions that make a system of closed regions particularly advisable for the conservation of fresh-water mussels may be briefly mentioned:
1. It has been previously stated that some of the mussels are spawning, or with spawn, during any period of the year. Many of the most important species are spawning during the late spring, early and mid summer; other equally important species form their eggs in the late summer, when they become fertilized and develop into the glochidium stage, but the mother clam retains them in marsupial pouches within her shell during the entire winter and even into the summer. All species of mussels carry the eggs in the marsupial pouches during the process of development to the glochidium stage or longer, whether the period be for a few weeks or for a few months. In this condition the mussels are said to be gravid. It is readily observed that when gravid mussels are disturbed they frequently discharge the young, regardless of whether these are mature enough to be liberated from the parent or not; certain species, such as the niggerhead, are particularly likely to do this.