122 PALEONTOLOGY OF THE EUEEKA D1STKICT. 



Strophodonta Calvini Miller. 

 Plate xiii, fig. 6. 



Strophodonta quadrata Calvin, 1878. Bull. U. S. Geol. Survey, vol. iv, p. 728. Not 



of Swallow, 1860. Trans. St. Louis Acad. Sci., vol. i, p. 639. 

 Calvini Miller, 1883. Cat. Amer. Pal. Foss., 2d ed., p. 298. 



The author describes this species as follows: 



"Shell small, concavo-convex, quadrate in outline. Cardinal extremi- 

 ties sometimes abruptly produced, sometimes rounded. Ventral valve very 

 convex, flattened on the umbo.and descending rapidly to the lateral and 

 frontal margins. Dorsal valve concave, following closely the curvature of 

 the other. Hinge area common to both valves, wider on ventral, finely 

 striated. Foramen only sufficiently developed to receive the extremities of 

 the bifid cardinal process. Muscular scars faintly impressed, not definitely 

 bounded. 



"Surface of ventral valve ornamented by fine radiating striae. From 

 three to five very small striae are implanted between pairs of more promi- 

 nent, but very slender, filiform, and often slightly interrupted ones. A 

 broad, shallow mesial sinus sometimes occupies the front half of the valve. 

 On the dorsal valve the striae are subequal, corresponding to the finer ones 

 of the ventral valve. 



"Length, 9 mra ; width, ll mm ; convexity, 5 mm ." 



The species occurs in the Devonian formation of Iowa and corresponds, 

 from the description, very closely with the Nevada form. The latter is pro- 

 portionally a little broader and slightly convex on the umbo, but does not 

 appear to differ specifically. 



Formation and localities. Lower horizon of the Devonian limestone, 

 Lone Mountain, 18 miles northwest of Eureka, and Comb's Peak, Eureka 

 District, Nevada. 



Genus CHONETES Fischer. 



It is difficult to determine the specific relations of many of the forms 

 of the Productidae and especially those belonging to the genus Chonetes, 

 as the shells are usually small and depend largely on the surface characters 

 and the presence of the cardinal spines for their specific determination. 



