_Body_, contractile, not much depressed, and slightly tapering.
_Buckler_, pustulous, trilobate, cheeks protuberant, with oblique,
annular, oculiferous tubercles.
_Abdomen_, with fourteen articulations, not lobate, the ribs double.
_Tail_, suborbicular, not so large as the buckler, covered with an
epidermis.
This genus derives its name from two Greek words, which signify
double ribs; many of the trilobites are thus characterized; but in no
species, is this organization so remarkable as in those which belong
to the Genus Dipleura. The expansion of the tail resembles, in some
degree, that of the Isotelus, but other obvious characters sufficiently
distinguish it from that interesting genus. The fossils arranged under
this section are larger than most other trilobites.
Dipleura Dekayi. _Green._ Casts No. 30, 31, and Figs. 8 & 9.
Clypeo lunato punctato; abdomine quatuordecim articulis duplicibus vix
lobatis; cauda suborbiculari; limbo lato convexo integerrimo; oculis
oblique deflexis.
The buckler is subtriangular, and covered with granulations; the
anterior portion of our specimen being mutilated, we cannot determine
its form exactly. The cheeks are very prominent, and swell up gradually
towards the oculiferous protuberances, which are oblique, and marked at
their apex with a depression, so as to give them an annular appearance.
The abdomen is crossed by fourteen double distinct articulations,
not interrupted in their course, by the two longitudinal furrows,
so common in most of the trilobites; but owing to certain curves or
irregularities in the ribs near their lateral termination, a trilobate
appearance may in some specimens be detected. Tail suborbicular,
convex, and covered with a thick epidermis.
The specific name of this species was given in compliment to Dr. James
E. Dekay, of New York, whose valuable paper on the genus Isotelus,
first directed my attention to the American trilobites.
The D. Dekayi has been found in several districts of the United States;
at Lockport in the State of New York, it is not uncommon. The small
specimen from which our cast of the abdomen and caudal end was taken,
is in the fine cabinet of Mr. William Hyde, who permitted me to use
it with his wonted liberality and kindness. It is said to have been
found in Northumberland, Pennsylvania, and occurs in grey carbonate
of lime. In the Philadelphia Museum, there is a fine fragment of this
species, in which there is embedded some crystals of iron pyrites; it
was obtained in Ulster County, New York. In the cabinet of the Academy
of Natural Sciences, there is a longitudinal and hollow fragment,
filled with ochre, and the oxide of iron; it is labelled from Lockport,
New York. At Mount Hope Institution, near Baltimore, there is also a
good specimen from the same locality. In the _Clinton collection_,
owned by the Albany Institute, there is a large extended fragment,
nearly five inches long. It is embedded in brown limestone, and was
found in Madison County, N. Y. There are twelve articulations of the
abdomen remaining, and the epidermal covering of the tail is distinctly
marked with numerous dots. In the same collection there is another
large fragment of this species, consisting of the tail and fourteen
articulations. It was found in Steuben County, New York; and occurs
in grey limestone. It is slightly contracted and very much depressed
laterally. There is also a head in the same kind of limestone, from
Cazenovia, Madison County, New York. In the cabinet of the Institute
there is another specimen of this species, about six inches in length,
and nearly perfect; it is also embedded in a similar rock, and was
brought from Rochester, Munroe County, New York.
The original of the head from which our cast was made, is in the
cabinet of P. A. Browne, Esq., and was found by that enterprising
geologist near Lehighton, Pa.