222 
A ug. F. Foerste 
magnified, are herewith presented. These illustrations indicate 
that Orthis fissi costa is more nearly related to Orthis triplicatella^ 
Meek, than at first supposed. In fact, Orthis triplicatella can 
scarcely be regarded as a distinct variety of Orthis fissicosta, Hall. 
Plectorthis (Eridorthis) nicklesi, nov. sp. 
{Plate IV, Figs. 3 A, B, C, D.) 
Shell small, usually about 15 millimeters in width, and ii mil- 
limeters in length, and with an estimated thickness varying between 
3.5 and 4.5 millimeters. I| 
The initial stages of the convex brachial valve begin with a ! 
median groove bordered by the first radiating plications. This | 
median groove remains a distinctive feature of the mature shell | 
and may be traced to a distance of 4 millimeters from the beak. I 
Anterior to this, the various plications intercalated along the mid-j 
die of the shell rise so as to form a median fold, often very distinct |j 
anteriorly, but sometimes only very slightly elevated. Each oft 
the primary plications bordering immediately upon the initial I 
median groove bifurcates very near the beak, and the inner onel; 
of these bifurcations usually forms part of the median fold ante- 
riorly. Four or five of the plications forming the fold originate at 
or posterior to the middle of the shell. Sometimes several smaller 
plications are added anteriorly. Ten or eleven lateral plications 
originate on each side of the median fold posterior to the middle : 
and to these plications others are added before reaching the mar- 1 
gin. The result is a rather finely plicated shell. Distinct and i 
rather distant concentric striations are easily recognized under a | 
lens. Shell thin, with the radiating plications showing distinctly! 
on the interior surface, and practically with no indication of the | 
muscular scars. The median elevation which usually separates! 
these scars, however, is plainly indicated. Posteriorly, it fills thei 
lower part of the space between the crural plates, and supportsij 
the thin cardinal process, which thickens moderately anteriorly. | 
Pedicel valve beginning with a median plication which is noti 
more prominent than the lateral primary plications. Beginning 
near the middle of the valve, the median part of the shell becomes j 
depressed anteriorly, and forms a rather shallow sinus, including ! 
a smaller number of plications than those found on the median, 
fold of the brachial valve. The pedicel valve usually is more 
