BRACHIOPODA. 251 



Diagnosis. Shells transversely subsemicircular or semielliptical; greatest 

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

 radiating strise 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 deltidium. 

 The teeth are widely divergent and are supported by plates which are produced 

 into elevated ridges nearly surrounding 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 witli 

 growth ; it has a much narrower cardinal area and the delthyrium is rudiment- 

 ary or incomplete. Dental sockets 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 by 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-plate 

 separating two large adductor scars, in front of which are two narrow elongate 

 impressions. Vascular and ovarian markings frequently well defined. Shell- 

 substance fibrous, strongly punctate. 



Type, Strophomena rugosa Rafinesque (de Blainville), lS2rJ,=^Lep((ena plamim- 

 bona, Hall, 1847, as recognized by King and other authors. 



Limiting the term Strophomena by this strict construction, it will prove to 

 be a genus of comparatively moderate vertical range, appearing so far as known 

 not before the age of the Trenton (Leptcena filitexta. Hall, Strophomena Thalia, 

 Billings, S. fluduosa, Billings, S. subtenta, Conrad), and becoming quite abundant 

 in the Hudson River group (Leptana planumbona. Hall, L. planoconvexa. Hall, 

 Strophomena sinuata, Emmons, Hemipronites nutans. Meek, Strophomena Wisconsin- 



