44© 



STUART WELLER 



generic value in these shells are to be found in the rostral portion 

 of the brachial valve. Six and possibly seven good generic groups 

 have been recognized, and will be defined here. 



Dielasma King 



Description. — Shell terebratuliform. Pedicle valve with or 

 without a median sinus, the beak strongly incurved, the foramen 

 large and encroaching upon the umbonal portion of the valve; 

 internally with well-developed dental lamellae. Brachial valve 

 usually without mesial fold; internally the crural plates are sepa- 

 rate from the dental socket-plates, they diverge from the apex of 

 the valve with an elongate attachment to the inner surface of the 

 valve, the free portion of the brachidium is short, with diverging 



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Fig. i. — A series of fourteen cross-sections (X2I) of the rostral portion of the 

 brachial valve of Dielasma formosa Hall. 



descending lamellae; between the crural plates for the full length 

 of their attachment to the inner surface of the valve, is a concave, 

 transverse plate for muscular attachment, which joins the inner 

 surface of the crural plates a little above their base; this plate 

 rests against the inner surface of the valve along the median line 

 for a portion or the whole of its length, or it may be free throughout; 

 when attached along the median line a pair of slender cavities, 

 triangular in cross-section, converge from the general cavity of 

 the shell toward the beak, but when the transverse plate is not 

 attached along its median line there is a single, broad and low 

 cavity beneath the plate, extending toward the apex; anteriorly 

 this plate extends to a greater or less distance beyond the attach- 

 ment of the crural plates and is pointed, rounded, or emarginate 

 in front; its surface is marked by concentric wrinkles parallel with 



