300 PAL5:0NT0L0GY 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 live 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 i« the 

 narrow marginal rim. The directioYi 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 pentamerus 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 

 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." MonogrBph, p. 87. 



