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, Leptana 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. 
