338 THE PALEONTOLOGY OF MINNESOTA. 
LLingula. 
Nepionic, Hyatt. ^ The smooth shell stage succeeding the protegulum, 
. Nealogic, Hyatt. Youthfulness, or the stage in which specific characters begin to 
develope. 
Ephebolic, Hyatt. The mature shell. 
Geratologic, Hyatt. Old age. It is indicated in many species of brachiopods by 
extreme thickness of the valves, obesity, or by numerous, crowded 
growth lines near the anterior margin, a condition which sometimes 
produces truncation and absence of striae at the margin. 
Class BRACHIOPODA, (Cuvier) Diimeril. 
Subclass LYOPOMATA, Owen. 
Order ATKEMATA, Beecher. 
Family LENGULTD.^], Gray. 
Genus LINGULA, Bruguiere. 
1789. Lingula, BliVGiKRK. H istoi re naturelle des Vers Testacy's. 
1892. Lingula, Hall. Pal;Bontology of New York, vol. viii, pt. i, p. 2. 
Description: "Shells subequivalve, equilateral; elongate-ovate, subquadrate 
or subtriangular in outline; broad over the pallial region, cardinal slopes more or 
less conspicuous; slightly gaping at both extremities. Brachial or dorsal valve 
somewhat the shorter, and with a slightly thickened hinge-line. Surface of the 
shell smooth, or concentrically and radiately striated. Animal attached by a long, 
muscular pedicle protruding from between the beaks of the two valves. 
"Muscular impressions numerous, but usually indistinct. In the recent species 
they are twelve in number upon each valve, and are somewhat unsymmetrical in 
their arrangement. They may be designated as follows : The umhonnl impressions, 
produced by a single muscular band passing directly across the cavity of the shell 
near the beaks, and by their contraction opening the valves; the lateral impressions, 
which are produced by three pairs of muscles, the anteriors passing from near the 
lateral boundaries of the visceral area on the pedicle [ventral) valve, forward to the 
anterior extremity of this tract on the brachial [dorsal J valve; the middles passing 
in just the opposite direction, from the anterior region of the pedicle-valve to the 
lateral region of the brachial; the externals passing from the ante-lateral region of 
the pedicle valve to the post-lateral region of the brachial valve; these muscles 
serving to move the valves forward and backward. The central impressions are 
* Values in classification of tlie stages of growtli and decline, with propositions for a new nomenclature," by Alpheus 
Hyatt; Am, Nat., vol. xxii, p. 872, 1888. Also " Genesis of the Arietidic." Mem. Mus. Comp. Zool., vol. xvi, no. 3, 1889. 
