cragin.] DESCRIPTIONS OF SPECIES. 69 
CRASSATELLID^. 
Genus PTYCHOMYA Agassiz. 
Ptychomya stantoni sp. n. 
PI. XII, figs. 4-6. 
Shell small, compressed, cuneate-ovate or trapezoid-ovate, a little 
excavated at the front of the beaks; the anterior side short, rounded; 
the posterior side long; the dorsal margin having a gentle and nearly 
straight descent back of the beaks for a considerable distance and 
then declining more rapidly, so that the posterior side of the shell 
is obliquely truncate above; the base nearly straight in its median 
and major portion; the posterior umbonal slope forming with the 
outer slope a gentle shoulder; valves thick, their margins crenulated 
within: beaks very little salient; lunule cuneate-lanceolate, deeply 
excavated; surface ornamented with a considerable number of ribs 
which form an anterior nearly vertical series of upwardly directed 
chevrons below the beaks, and a posterior oblique series of nearly 
closed, distally directed angles whose apices lie on the umbonal 
shoulder; on the posterior part of the shell the ribs pursue a nearly 
straight course backward and downward to the basal and posterior 
margins, though arranged in two converging sets, in each of which 
they are slightly divergent; on the anterior region they are subhori- 
zontal, curved forward and upward, and often minutely undulated 
or zigzagged. The sides of the subumbonal chevrons, exclusive of 
the part that forms the usually more or less cuspidate apex, diverge 
at an angle which is right or sometimes slightly acute in the earlier 
ribs and becomes quite obtuse in the later ones. The ribs are gen- 
erally linear-compressed and separated by broader concave intervals, 
but in occasional specimens, and especially on the posterior region, are 
more obtuse, with the intervals equaling or distally exceeding them. 
Measurements. — A nearly complete right valve indicates: Height, 
16.5 mm.; length, 31 mm.; breadth, 8 or 9 mm. An internal cast of 
a right valve gives: Height, 18 mm.; length, 32 mm. 
Occurrence. — Twenty-three specimens, in part only fragments, with 
the characteristic ornamentation, are in hand. All are from the 
locality H miles east of Malone station, and most, if not all, from the 
lower part of the Theta beds there outcropping. 
In pattern of ornamentation and in size Ptychomya stantoni comes 
nearest to Pt. complicata Tate, from which it differs in having the 
beaks less anteriorly placed and the posterior truncation very oblique 
instead of nearly square; the position of the beaks and the line of 
chevrons below them is as in Pt. koeneni Behrendsen. The general 
form has some resemblance to the latter species, but its lateral profile 
departs more from a regular oval and tends toward a cuneate. It 
