While on this subject, it may be useful to the student to know that a brachiopod differs from ordinary bivalves, mussels, cockles, etc.
"The Student's Elements of Geology"
Sir Charles Lyell
The genus Lingula, a small shell, traces its ancestry back nearly to the base of the Ordovician system of rocks, an almost inconceivable lapse of time, while one species of brachiopod shell endures unchanged from the Trenton Limestone to the Lower Carboniferous.
"Animals of the Past"
Frederic A. Lucas
The two valves of the brachiopod shell are unequal in size, and in each valve a line drawn from the beak to the base divides the valve into two equal parts.
"The Elements of Geology"
William Harmon Norton