600 Forty- fifth Report on the State Museum. 



deep, and continued as narrow grooves or indentations across 

 the cardinal area. The crural plates are extended laterally with 

 a slight curve, but are not supported hj septa ; at their inner 

 margins they unite to form a callosity, upon which rests 

 the short, bilobed cardinal process, which scarcely extends 

 beyond the hinge-line. The muscular surface of this process is 

 cordate in outline and is placed at a low angle to the plane of 

 the area. A low median ridge extends forward from the hinge- 



Figs. 273, 274. — Strophomena rugosa. After de Blainville. 



plate separating two large adductor scars, in front of which are 

 two narrow elongate impressions. Yascular and ovarian mark- 

 ings frequently well defined. Shell-substance fibrous, strongly 

 punctate. 



Type, Strophomena rugosa Eafinesque (de Blainville), 1825,= 

 Leptcena planumbona, Hall, 1847, as recognized by King and 

 other authors. 



Distribution, Lower — (?) Upper Silurian. 



Orthothetes, Fischer de Waldheim. 1830. 

 (Plate 14, Figs. 8-16.) 

 Shells plano-convex or biconvex, sometimes becoming concavo- 

 convex with age. Brachial valve usually the deeper in the 

 pallial region; pedicle-valve highest about the umbo, which is 

 sometimes much extended, and often shows a pronounced ten- 

 dency to irregular growth. Surface covered with slender, 

 subequal radii, which are crenulated by sharp concentric striae. 

 Hinge-line long, equaling, and often greater than the width of 

 the shell; in some species frequently showing a tendency to 

 auriculation at the extremities. Pedicle-valve with the cardinal 

 area prominently developed ; the delthyrium covered by a thick, 

 more or less convex deltidium, which is rarely if ever perforated 

 at maturity. On the interior the teeth are moderately huge, 

 sometimes thickened, but not supported by dental plates. The 



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