55 
the cardinal angles; delthyrium slightly higher than wide. Median sinus 
moderately well developed; beginning at the beak as a simple furrow 
bounded by two plications much stronger than any others upon the valve 
it has implanted within it a single median plication, which remains simple; 
about the same time the bounding plications bifurcate unequally sending 
the weaker branch into the sinus. The adult shell has usually these three 
simple plications, but at times the bounding plications again divide un- 
equally and send the smaller branch into the sinus giving to ft five plications, 
which at the anterior edge of the valve are usually almost as strong and 
subangular as those upon the rest of the shell. Lateral plications 8 to 12, 
usually abruptly elevated, at times gently rounded; when no bifurcation 
occurs the strength of the plications decreases from those bounding the 
median sinus to the lateral extremities; usually the plication bounding the 
sinus bifurcates in the umbonal region, and often the one next it also, 
these four plications are then usually distinctly narrower than the next 
two or three out over the lateral slopes; these successive bifurcations of 
the plications bounding the median sinus, sending one to two branches 
into the sinus and one upon the lateral slope, result in their lessening 
prominence until they become equal in width, and often in height, to the 
median plication of the sinus; the other plications upon the lateral slopes 
are simple. 
Brachial valve less convex than the pedicle; beak small, closely in- 
curved. Median fold well defined from the beak where it originates as 
a single plication; this quickly bifurcates and these two plications continue 
strong and equal to the front of the valve, forming a level top to the sharply 
elevated fold; these usually bifurcate unequally but once, sending the 
smaller branch down the slope, resulting in four plications; at times two 
more are added, making six. Plications of lateral slopes similar in number, 
angularity, and bifurcation to those of the pedicle valve, though bifur- 
cation is usually more forcibly expressed here than upon the opposite 
valve. Shells with the lateral plications bifurcating are much more 
numerous than those with simple plications. 
The finer surface markings consist of concentric growth lines which 
are usually fine, but at times, especially towards the front of the shell, 
become lamellose. The entire surface is also covered by fine radiating 
lines which often become radiating rows of pustules; these radiating lines 
are usually equal in strength to the concentric, frequently they are the 
more conspicuous. 
Remarks . This species, S. boonensis, and S. opimus are discussed 
below under “General Discussion of Spirifer rocky montanus.” 
It is at times rather difficult to distinguish this species from S . centro - 
natus. The typical rockymontanus has, however, fewer, broader, and 
more angular plications than the typical centronatus . Comparing speci- 
mens of equal size, and taking the measurement 15 mm. from the beak, 
a specimen of rockymontanus from section 2-12 has 11 lateral plications 
in 15 mm. upon the convex part of the shell, whereas a centronatus from 
2-23 had 18 plications in the same distance, and one from 2-25 had 20 
plications. The latter species likewise lacks the bifurcating lateral 
plications and radiating pustules present upon so many of the former. 
