740 THE PALEONTOLOGY OF MINNESOTA. 



lEntTiuurus rarlcostatus. 



are deflected abruptly, each segment terminating in a 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 



Fig. 57. Pygidium of Enerinurus 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 pleurae 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. 8410V, 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. 



(?) ENCBINURUS RARICOSTATUS Walcott, 1877. 



cf. Enerinurus raricostatus WALCOTT, 1877. Adv. sheets, Thirty-first Kept., N. Y. State Mus. Nat. 



Hist., p. 16. 



Enerinurus raricostalus WALCOTT, 1879. Thirty-first Kept., N. Y. State Mus. Nat. Hist., p. 69. 

 Enerinurus raricostatus SAFFOBD and VODOES, 1887. Proc. Acad. Nat. Sci. Phil., p. 167. flg. 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 E. 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 Sattbrd and Vogdes, of a specimen from Lebanon, Tenn. 



