2(M» TJii Auk ri<-iiii (jeoJogixt. October, 1891 
Valves t'lon<;ale. hoat-shapod. extremities acuminate, suhecjiial 
the anterior spine strong, projecting greatly beyond the end of the 
hinge margin: from this sjjine a sharply defined thin, and gently 
curved dt)rsal ridge or carina, extends ))ackward to and a little 
bej'ond the angular postero-dorsal extremity. A sharp impression 
extends a short distance obliquely downward from this extremity 
of the ridge. Hack flattened except where it runs into the ter- 
minal spines; a faint channel- along the edge. Ventral edge 
nearly straight, curving uniformly upward at each end. Surface 
without ornamentation, with point of greatest convexity ( thick- 
ness ) at the dorsal carina al)out midway of the length. 
In a view of the inner side the dorsal edge is strongh' convex, 
and the anterior junction with the ventral edge very acute and oc- 
curring beneathor about the middle of the higlit. The largest and 
best of the valves seen presents a feature about which I could 
not satisfy myself that it really belongs to the valve. I refer to 
the thin oblique diaphragm-like structure shown in the right half 
of fig. 2. If it is reallv a normal structure then it appears to be 
restricted to the right valve, as I fail to notice any sign of it in a 
well preserved left valve. Nor can I detect any sign of such a 
diaphragm in the other species. 
The elongate form, sharph' carinate back, acute posterior end, 
and the strength of the anterior spine are the distinguishing 
features of this species. It should recpiire l)ut a glance to sep- 
arate it from all of its known associates. 
Formation and locality: Lower Helderberg irroup, Albany county, N. Y. 
Types in Dr. C. E. Beecher's collection. 
Beecherella subtumida, n. sp 
PLATE II, FIOS. 5-7. 
Size of right valve: greatest length. I.IM) mm. ; greatest hight. 
0.70 mm.; thickness, 0.55 mm.; length of A'alve at inner edge, 
1.53 mm. ; projection of spinelieyondanterior edge about 0.35 mm. 
Valves elongate-ovate, slightly widest in the po.sterior half; 
extremities, excluding the anterior spine, subequal. the jjosterior 
a little the bluntest of the two. Dorsal margin straight, sharply 
rounded at the postero-dorsal angle, and very slightly l)ent down 
where it passes into the strongly projecting anterior spine. 
Antero-ventral margin (in a side view) straight or faintly concave 
to the end of the spine. Ventral edge, uniformly convex, from 
