26 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
species, HL. deltoideus Shumard and E. nereus Hall, have as many as 
eight pairs of ribs. Most of the species have seven pairs, one, FE. 
americanus Vogdes, has six, and E. elegantulus Billings has only five. 
The species may be taken up in alphabetical order. 
E. americanus Vogdes (Description new Crustacea from Clinton of 
Georgia, 1866, p. 1), has only six pairs of ribs on the pleura, and no 
pustules, thus ruling it out at once. 
E. deltoideus Shumard (Geol. Missouri, 1855, p. 198, pl. B, f. 10), is 
similar in many ways to E. reflexus, having the rings very numerous, 
twenty-four in number, and interrupted by a smooth lane. There 
are, however, no pustules except very indistinct granules, on this lane, 
and the ribs, of which there are eight pairs, are likewise smooth. 
E. egant S. A. Miller (Journ. Cine. soc. nat. hist., 1880, 2, p. 254, 
pl. 15, f. 1, 1b), has a long terminal spine, only seven pairs of ribs, 
which are narrow with wide interspaces, and while the axial lobe of 
the pygidium is on the same plan as in EL. reflexus, it has fewer rings 
and tubercles. 
E. elegantulus Billings (Cat. Silurian fossils Anticosti, 1866, p. 62), 
lias only five pairs of ribs, the median lane on the axial lobe is without 
tubercles, but the first eight rings cross it. 
E. indianaénsis Kindle (28th Ann. rept. Dept. geol. and nat. res., 
Indiana, 1904, p. 482, pl. 24, f. 14, 15), is a very peculiar species, not 
at all of the same type as the one under discussion. It has fifteen 
rings on the axial lobe and ten pairs of ribs. Each rib crosses the axial, 
lobe and has three to five tubercles. 
I. nereus Hall (20th Rept. N. Y. state cab. nat. hist., 1868, p. 375, 
pl. 21, f. 15) has no flattened lane along the top of the axial lobe, and 
no nodes on the rings or ribs. 
E. ornatus Hall and Whitfield (Pal. Ohio, 1875, 2, p. 154, pl. 6, f. 16) is 
quite similar to the present species, but has only seven pairs of ribs, and 
the ribs themselves are narrower and the spaces between them wider. 
There are also only twenty rings and five nodes on the axial lobe. 
E. thresheri Foerste (Bull. Sci. lab. Denison univ. 1887, 2, p. 101, pl. 
8, f. 26) is a small species which is similar to the last and to FE. reflexus. 
There are, however, only seven pairs of ribs, which are themselves 
exceedingly narrow; there are also only eighteen rings on the axial 
lobe, and six pustules on the smooth lane. 
E. tuberculifrons Weller (Bull. Chicago acad. sci.. 1907, no. 4, pt. 2, 
p. 259, pl. 24, f. 12, 13) is a small form with a short wide pygidium 
which is without nodes and the rings of which cross the axial lobe 
without interruption. 
Formation and locality: — From the Niagaran at Wauwatosa, Wisc. 
