Si/iin'au Strata in WjidiiiiiKj and So»th hahnta. — Beeclicr. 31 
ing in front of the glabella. Posterior marginal fold wide at the outer 
end, shield narrower here than opposite the cheeks. 
Thorax not known, but the i-elief of the approximate margins of the 
two shields indicates a prominent rachis and strongly geniculated 
pleura, with furrows directed forward. 
Pygidium longer in proportion than the head-shield and more decid- 
edly arched. Rachis nearly three quarters of the length of the shield: 
it has about six joints, the anterior four prominent and each crowned 
with a tubercle. Side lobes sloping down to a flattened marginal fold, 
which is strongly geniculated at the front, with an almost vertical outer 
slope. 
Sculpture. Surface of both shields minutely granulate. 
Size. Length of pygidium 3 mm.: width 4 mm.: the head shield is 
somewhat shorter. 
Horizon and localiti/. Limestone of the Oleuellus zone at Troy, N. 
York. Collected by Mr. Chas. Schuchert. 
A comparison of the young or larval forms of 3£. pnlcheUus 
or M. puuctatus witli the adult of this species is quite instruc- 
tive and shows how this type has changed from the tiine of 
its api)earaiice in tlie Olenellus zone until its extinction in tiie 
P, davidis sub-zone. Especiall}^ do we see how the antei'ior 
axial furrow originated on a flattened margin, first with a 
small pit in front of the glabella, as in M. schucherti, then 
passing to a furrow as deep as the dorsal or tlie marginal fur- 
rows, and then in the tumid species, when the whole shield be- 
comes strongly arched, this axial furrow becomes mucli 
shallower and tends to disappear. 
Comparing with Agnf)stus, the pit in M. schneherfi in front 
of the glabella is parallel to the sharply indented furrow of .1. 
^tissus., and while in the thick-shelled sections Limbati, Parvi- 
f routes and Laevigati, of Agnostus this furrow is effaced, in 
the thin-shelled Longifrontes. or at least the majority of them, 
tliis furrow being necessary to stiffen the front of the sliield, 
is retained. 
ON THE OCCURRENCE OF SILURIAN STRATA IN 
THE BIG HORN MOUNTAINS. WYOMING, AND 
IN THE BLACK HILLS, SOUTH DAKOTA. 
By C. E. HEECHf:R, Ni'w Haven, Cumi. 
At the request of W. H. Weed of the United States Geologi- 
cal Survey, the writer herewith makes a brief statement re- 
garding two western localities of Silurian, oiu^ of which, so 
far as known, has nof been liitiierto noticed. Theii- chief 
