360 
PALAEONTOLOGY OF NEW-YORK. 
ward, and sometimes a little more prominent or almost ncdose in the 
middle. Each of the lateral lobes is marked by fourteen or fifteen (and, 
in one example, sixteen) ribs ; the anterior ones very regularly arching, 
while about four or five of the posterior ones are turned backwards, 
approaching the parallel of the axis. Each rib is marked by a narrow 
groove along its summit, continued to where the ribs coalesce in the 
narrow marginal rim. The direction of this suture, near the origin of 
the ribs, is a little below the middle, but, in its course, approaches 
more nearly the upper margin. 
Surface granulose, with a row of stronger granules or small pustules on 
each side of the furrow marking the ribs, and still stronger ones on 
the middle of the annulations of the axis. 
I have a single head which is more convex, and the frontal lobe of the glabella 
more prominent than those referred to the preceding species, and which may belong 
to this one. 
Until recently, I had regarded the numerous specimens of the pygidium occurring 
in the pentamems and shaly limestones of the Helderberg group as belonging to 
one species, presenting some variety in the number of annulations ; but a com¬ 
parison with the original specimen of D. micrurus described by Dr. Green, shows 
that it has a more rigid aspect, is less curved outwards, and is proportionally 
narrower on the posterior half of the pygidium, and the axis is proportionally 
longer and more rigid ; while in specimens which have not suffered pressure, the 
sides are more abruptly bent downwards to the margins. These forms, whether large 
or small, have shown usually twenty articulations of the axis and fourteen or fifteen 
s ribs in the lateral lobes, without any evidences of gradation in number which would 
unite the preceding species. 
Dr. Green describes the original as having “ eighteen articulations of the tail and 
abdomen.” The specimen, which is a cast, measures more than two inches in length*, 
and has the first narrow articulating ring obliterated, while the posterior part of 
the axis is so much worn as to obscure the annulations. At the same time twenty 
annulations may be traced, and there has probably been one more; while there are 
sixteen ribs on one side, and on the other side two of the anterior ribs are broken 
off. 
The species is cited by Dr. Green as from Trenton falls; but not only is the 
limestone of a different character, but the associated fossils prove very satisfactorily 
* “ Length, two inches and a half.” Monograph, p. 57. 
