CERATJRUS. 5 7 
at this horizon, and as the specimens obviously do not belong to Isotelus or Cryptolithus, 
it is probable that they were formerly part of a Calymene. 
All the endopodites are of chitinous material, and the various specimens show, accord- 
ing to the perfection of their preservation, from four to six segments. The endopodite as 
a whole tapers but slightly outward, and the individual segments are of nearly equal length. 
They appear to be but little crushed, and are oval in section, with a crimped anterior and 
posterior margin. One or two show a median longitudinal ridge, such as is seen in some 
appendages of Triarthrus. Each segment is parallel-sided, with a slight expansion at the 
distal end, where the next segment fits into it. 
Under the heading "Ordovician Crustacean Leg," Walcott (1918, p. 154, pi. 36, figs. 
1, 2) has recently redescribed these specimens, and thinks that they do not belong to Calym- 
ene, nor, indeed, to any trilobite. He concludes that they were more like what one would 
expect in an isopod. Passing over the fact that the oldest isopod now known is Devonian, 
the fossils in question seem to me quite trilobite-like. Walcott says : 
The legs are associated with fragments of Calymene meeki but it is not probable that they belong to 
that species ; if they did, they are unlike any trilobite leg known to me. The very short coxopodite and 
basopodite are unknown in the trilobites of which we have the legs, as they are fused into one joint forming 
the long protopodite in the trilobite. The distal joint is also unlike that of the trilobite legs known to us. 
A great deal of Doctor Walcott's difficulty probably arises from his homology of the 
coxopodite of the trilobite with the protopodite of the higher Crustacea. The coxopodite 
of the trilobite is not fused with the basipodite, this latter segment always remaining free. 
Indeed, Walcott himself says of Neolcnus (1918, p. 128) : 
Each thoracic leg (endopodite) is formed of a large elongate proximal joint (protopodite), four strong 
joints each about 1.5 times as long as wide (basopodite, ischiopodite, meropodite and carpopodite) ; two 
slender elongate joints (propodite and dactylopodite) and a claw-like, more or less tripartite termination. 
Walcott's drawing (pi. 36, fig. 1 ) is a composite one, and while it shows eight seg- 
ments, I was not able to count more than seven on any of the specimens themselves. In 
regard to the terminal segment, the dactylopodite of the limb shown in his plate 36, figure 2, 
is unusually long, and a comparison with other photographs published on the same plate 
shows that such long segments are unusual. 
Proof that these are appendages of a Calymene is of course wanting, but there is no 
particular reason so far to say that they are not. 
Measurements: Two of the more complete specimens, each showing six segments, are 
each 8 mm. long. 
Somewhat similar to the specimens from Covington are the ones described by Eich- 
vvald ( 1825, p. 39, i860, pi. 21 ) , the specimens being from the Silurian of Gotland. The figure 
copied by Walcott (1881, pi. 6, fig. 4) has never been looked upon as entirely satisfactory 
evidence of the nature of the specimen, and so far as I know, the fossil has not been seen 
by any modern investigator. 
Ceraurus pleurexanthemus Green. 
(PI. 11 ; text figs. 12, 17-19, 21, 22, 24, 29, 30.) 
Illustrated: Walcott, Ann. Lye. Nat. Hist. New York, vol. 11, 1875, pi. 11; — 31st Ann. Rept. New York 
State Mus. Nat. Hist., 1879, pi. 1, fig. 3;— Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pi. 1, figs. 
1-5; pi. 2, figs. 1-4, 6-8; pi. 3, figs. 2, 4-7; pi. 4, figs. 1, 2, 4-6, 8; pi. 6, fig. 3; — Smithson. Misc. Coll., vol. 67, 
