BRACHIOPODA — TELOTREMATA. 



355 



418. S. trinucleus Hall. (Fig. 465.) Mississippic. 

 Differs from the preceding in having a depression in the center 



of the fold, and in the pronounced margins of the sinus, which 

 sometimes form distinct ribs near the front. 



St. Louis of Indiana, Illinois, Missouri and Kentucky. 



419. S. argentea (Shepard) (S. subtilita Hall). (Fig. 466.) 



Carbonic. 



Subovate, usually longer than wide, moderately gibbous. Fed- 



icle valve slightly more convex than the brachial with prominent 



beak ; mesial sinus becoming obsolete about the middle of the 



Fig. 466. Seminula argentea, one specimen with Crania modesta (No. 61 ) attached 



(Ind. Geol. Survey). 



shell ; a more or less distinctly impressed line usually extends 

 along the bottom of this sinus from beak to front. Brachial valve 

 with ill defined mesial fold. Surface marked with concentric 

 striae and faint traces of radiating lines. Average length about 1 

 inch. 



Throughout the Upper Carbonic of North America. 



420. S. dawsoni Hall and Clarke. (Fig. 467.) Carbonic. 



Smaller and more triangular than preceding, with fold and sinus 

 scarcely defined. Greatest width in anterior third of shell. 



Windsor limestone of Nova Sco 

 tia — extremely abundant. *^ 



Fig. 467. Seminula dawsoni. 

 (After Dawson.) 



CXXIV. Meristella Hall. 

 Oval to suborbicular. Valves 

 unequally convex, with or without 

 a faint median sinus or fold. Umbo 

 of pedicle valve greatly incurved at maturity so as to conceal the 

 open delthyrium. No cardinal area or spondylium present. Sur- 

 face smooth or with fine concentric lines and very fine radiating 



