836 Report oF THE STATE GEOLOGIST. 
forming an umbonal pit. Cardinal process represented by two 
faint processes beneath the beak. 
Fias. 440, 441.—Hemithyris psittacea, Gmelin (sp.) a, pedicle-valve; b, brachial valve; c, pos- 
terior adductors; d, anterior adductors; e, diductors; f, pedicle muscles. (Davipson ) 
Type, Hemithyris psittacea, Gmelin (sp.). (Recent.) 
Distribution. Tertiary (?) — Recent. 
Acanthothyris, D’Orbigny. 1850. 
Shells like Hemirnyris, but having the surface covered with 
spines, and the dental plates well developed. 
Fia. 442.— Acanthothyris spinosa, Schlotheim (sp.). (Woopwarp.) 
Type, Acanthothyris spinosa, Schlotheim (sp.). (Jurassic.) 
Distribution. Jurassic— Recent. 
Syntrophia, Hall. 1898. 
(Plate 45, figs. 1-3.) 
Shells transversely elongate, biconvex, with straight hinge- 
line whose length nearly equals the greatest diameter of the 
valves; each valve medially divided by an open delthyrium, 
External surface smooth, with fine concentric lines visible only 
about the margins; the inner shell-layers show a strongly fibrous 
radiating structure without punctation. The pedicle valve bears 
a more or less clearly developed median sinus and the brachial 
valve a broad, indistinct fold. 
On the interior the teeth are very small, lying at the extremi- 
ties of the delthyrial margins and supported by dental plates 
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