740 THE PALEONTOLOGY OF MINNESOTA. 
(Enecrinurus raricostatus. 
are deflected abruptly, each segment terminating ina broadly obtuse extremity. 
From the line of geniculation they are bent abruptly backward. The segments are 
simple throughout, or with but very faint sulci. 
Pygidium subpentagonal in outline; length and width equal. Axis relatively 
narrow, the lateral articulating surface sloping abruptly backward so that the outer 
lateral margin of the shield begins at a point fully two-thirds the length of the 
shield from the anterior margin. Post-lateral slope abrupt. Axis convex, with 6 or 7 
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Fig. 57.—Pygidium of Enecrinurus vannulus, x 3. 
rounded annulations, which extend entirely across, and behind these 8 or 9 more 
which are interrupted medially by a smooth area. The axis, which tapers rapidly, is 
continued beyond the annulations, its extremity reaching almost to the margin, 
and enveloped by an elevated oval ridge having the appearance of an adventitious 
pleural rib. The pleure bear six short, simple ribs which are elevated at their 
proximal extremities on the dorsal furrows and curve abruptly backward. The 
first three of these may end in free, blunt tips; the last three are confluent with 
the margin of the shield, the final pair enclosing the peculiarly enveloped extremity 
of the axis. Length and width of the typical specimen, 7 mm. 
Formation and locality.Lower blue beds of the Trenton limestone, Janesville, Wisconsin (Museum 
No. 8410); upper beds, Beloit, Wisconsin (Museum No. 8418). 
Of this species I have observed but three specimens: a pygidium and a part of the cephalon from the 
former locality, and a cephalon with ten segments of the thorax from the latter. The species probably 
approaches E. raricostatus Walcott (of which as yet but the pygidium is known) more nearly than 
any other American form. That species is said to possess from 13 to 16 smooth continuous annulations 
on the axis of the pygidium, and it is upon the difference of the two forms in this respect together with 
the additional knowledge of the other parts of the animals, that this proposed species is grounded. 
(2?) Encrinurus rAricostatus Walcott, 1877. 
cf. Encrinurus raricostatus WALCOTT, 1877. Adv. sheets, Thirty-first Rept., N. Y. State Mus. Nat. 
Encrinurus Pre ra ee 1879. Thirty-first Rept., N. Y. State Mus. Nat, Hist., p. 69. 
Encrinurus raricostatus SAFFORD and VODGES, 1887. Proc. Acad. Nat. Sci. Phil., p. 167. fig. 2. 
There is a single pygidium in the material before me, from the Trenton 
limestone at Mineral Point, Wisconsin (Museum No. 8403), the original locality of 
Mr. Walcott’s species, that has the axis annulated for most of its length and the six 
lateral ribs relatively larger, blunter at both extremities and with a less abrupt 
posterior curve than FH. vannulus. It agrees well with the original description 
which was based upon this part alone, and with the only figure yet given of the 
species, that published by Safford and Vogdes, of a specimen from Lebanon, Tenn. 
