BRACHIOPODA. 299 



ing lines which are crossed by stronger, rather regular concentric plications. 

 In the pedicle-valve the cardinal area is moderately high and the delthyrium 

 in its normal condition probably closed by a convex plate. The teeth are very 

 divergent and from their bases extend the elevated margins of two linguiform 

 muscular scars, traversing the shell for almost its entire length. These scars, 

 which may be regarded as the diductors, enclose two, much shorter, but still 

 elongate adductors. In the brachial valve the cardinal process is bipartite on 

 its anterior face, each of the lobes being grooved behind ; the crural plates are 

 very long and divergent, terminating in elevated extremities or crura. The 

 lower moiety of these plates is produced on each side of a strongly elevated 

 muscular ridge, curving slightly inward on the sides, then outward on approach- 

 ing the anterior margin of the valve, each branch recurving and passing back- 

 ward, parallel to the median axis, as far as the base of the cardinal process. 

 The symmetrical spaces thus limited are each divided transversely at about one- 

 third their length from the hinge-line, by a somewhat lower vertical ridge. 

 The four areas thus enclosed represent the posterior and anterior scars of the 

 adductor muscles. Between the inner muscular walls, in the median line, is a 

 low, rounded, longitudinal ridge. 



Type, Leptcena subquadrata, Hall. Lower Helderberg group. 



Under the foregoing diagnosis it is proposed to include a few peculiar species 

 which have usually been referred to Lept^na, of the type of L. transversalis 

 (= Plectambonites). While they resemble in many features the structure of 

 this group, there are important differences ; in the composition of the cardi- 

 nal process ; in the arrangement of the muscular scars, and in the surface 

 ornamentation. The most striking of these peculiarities are the great muscular 

 scars bounded by high walls. In the pedicle-valve the outer diductor scars 

 are much more elongated than ever in Plectambonites, and in the brachial 

 valve the adductors have the quadruplicate arrangement usually seen to the 

 best advantage in species of Orthis. In Plectambonites transversalis, however, 

 these adductor impressions, though greatly elongated, are nearly parallel to each 

 other, all converging toward, or meeting in the umbonal region. 



