Report of the Stats Oeolos 599 



the center of the ventral area there is often a Btrong process (fre- 

 quently bilobed). which extends beyond the cardinal line Mus 

 cular area of the pedicle- valve strongly marked, and Limited by 

 a prominent border. Brachial valve with ;i narrow hinge-area 

 transversely or longitudinally striate, or both, and marked in the 

 center by a deltidial scar. Cardinal process double, each divi- 

 sion notched or bidentate at the extremity; muscular areas 

 quadrangular. 



Type, Strophonetta fasciata, Hall. 



Distribution. Upper Silurian— Upper Devonian. 



Subgenus Amphistrophia, Hall. 1892. 

 (Plate 15, Figs. 12-14.) 

 Shells with eon vex deltidium and very short row of cardinal 

 denticulations. In the pedicle-valve the muscular area is not 

 inclosed ; in the brachial valve muscular markings extremely 

 faint. 

 (Type, .1. striata, Hall (sp.). Upper Silurian.) 



Strophomena, Bafinesque (de Blainville). 1825. 

 (Plate 14, Figs. 1-7. 1 



Synonyms; (?) Strophomenes, Bafinesque, 1820, 1S31 ; Defrance, 

 L824. 



Shells transversely subsemicircular or semielliptical ; greatest 

 width along the hinge-line. Surface concavo-convex and covered 

 with fine radiating striae which are equal or alternate in size. 

 The pedicle-valve is slightly convex about the umbo, but becomes 

 rapidly concave toward the middle with the apex perforated, 

 except in old age. The cardinal area is conspicuous and nearly 

 vertical, and the delthyrium closed by a convex plate or delti- 

 dium. Teeth widely divergent and supported by plates 

 which are produced into elevated ridges nearly surround- 

 ing the muscular area. The latter is relatively short, subcircular 

 in outline, deeply excavated and divided medially by a more or 

 less distinctly defined longitudinal ridge which is often continued 

 over the pallial region. 



The brachial valve is concave at the umbo, becoming strongly 

 convex with growth ; it has much the narrower cardinal area, and 

 the delthyrium is rudimentary or incomplete. Dental sockets 



151 



