812 Report or THE State GxEoLoGIsT. 
Zygospira, Hall. 1862. 
Synonym, Anazyga, Davidson, 1882. 
(Plate 40, figs. 13-25.) 
Shells usually small. Outline subcircular or transversely oval. 
Contour subplano-convex. Surface sharply plicate. Pedicle- 
valve with a median plicated ridge. Umbo narrow and promi- 
nent; beak acute and incurved. Foramen elongate, rarely 
apical, enclosed by the deltidial plates. Hinge-line long and 
straight ; cardinal extremities rounded. A distinct false area is 
formed by a pair of ridges diverging from the beak toward the 
Fie. 393. Fie. 394. 
Fie. 395. Fia. 396. 
Preparations of Zygospira modesta, (Say) Hall; showing the variation in position of the jugum. 
cardinal extremities. On the interior the teeth are moderately 
well developed and unsupported by dental lamelle. 
The brachial yalve is depressed convex in the umbonal region 
and bears a more or less conspicuous median sinus. The hinge- 
plate consists of two broad, stout processes, diverging outwardly, 
grooved on their summits, and separated from each other by a 
narrow, sharp cleft. They form both the socket walls and crural 
bases, and are supported by a low median ridge. Muscular 
impressions obscure in the typical species. 
The crura are short and straight at their union with the 
primary lamelle, making a rectangular curve. The first half- 
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