BRACHIOPODA. 191 



laminae or septa, near their junction with the hinge line, originate two broad thin brachial 

 olates ********** 

 * * * * * * * * * * 



"In the young shells of this species, the dorsal valve is abruptly incurved; the 

 beak of the ventral valve is nearly as high as the opposite, and has beneath it a distinct 

 triangular foramen and an apparent area * * As the shell grows older, the 



beak of the opposite valve fills the foramen; and the notch in the beak is carried farther 

 inwards as the shell thickens, so that in old shells the beak presents a somewhat circular 

 perforation, which communicates below with the triangular cavity. 



"In extremely young shells, there is no appearance of mesial sinus or elevation." — 

 Hall, 1859. 



Family Pentameridae 

 Genus Gypidula Hall 



Gypidula multicostata Dunbar 



Plate 48, figures 10-19 



Gypidula multicostata Dunbar, 1919, Tenn. Geol. Surv., Bull. 21, pi. 2, fig. 22. 

 Gypidula multicostata Dunbar, 1920, Trans. Conn. Acad. Arts and Sciences, vol. 23, p. 



131, pi. Ill, figs. 12, 13. 



Description — "Shell subtrigonal in outline. Ventral valve strongly convex and 

 much larger than the dorsal. Its beak narrow and strongly arched, but not incurved 

 over that of the opposite valve. The sides diverge regularly to their greatest width at 

 about three-fourths to four-fifths the distance to the front of the shell, then round 

 abruptly into the broad and very slightly emarginate anterior margin, so that the 

 outline is almost an isosceles triangle. Dorsal valve transversely elliptical and gently 

 convex. A broad but low flat fold occupies the median half of the ventral valve, becom- 

 ing obsolete before reaching the beak, and a corresponding sinus is present on the dorsal 

 valve. The fold and sinus bear each ten to twelve slender, sharp, angular plications 

 separated by angular grooves of equal width, while the inner part of each lateral slope 

 is marked by four or five obscure plications, of which the outer is the most indistinct. 

 Beyond these, the lateral slopes are smooth. The spondylium in the ventral valve 

 is similar to that of G. coeymanensis, but considerable variation may be noted in the 

 development of the median septum which supports it. In some specimens the septum 

 is strong and in a few entirely absent. 



"Dimensions — Length, 40 mm.; width, 40 mm.; thickness, about 25 mm. 



"Discussion — The species is readily distinguished from all other described ones by 

 the large number and slenderness of its plications. It is most closely related to G. 

 coeymanensis, but that species has fewer and much coarser plications and reaches its 

 greatest width nearer its mid-length." — Dunbar, 1920. 



Remarks — -A description of the Missouri specimens follows: 



Shell subtriangular in outline, anterior border emarginate, anterior line of junction 

 of valves sinuate. Ventral valve more convex than dorsal valve, elevated medially from 

 anterior margin posteriorly; beak prominent, incurved over that of dorsal valve. Dorsal 

 valve depressed medially from anterior margin posteriorly; beak incurved, fills delthy- 

 rium. Surface ornamented with radiating plications that become less prominent on 

 valves laterally, increase by bifurcation, and the more conspicuous of which extend 

 almost, if not quite, to beaks, crossed by concentric growth lines. Medial elevation of 

 ventral valve has 7 to 12 plications; medial sinus of dorsal valve has 7 to at least 9 

 plications. 



