CERAURUS. 57 



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 Calymcne. 



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 mecki 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 Neolenus (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. i) 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- 

 wald (1825, p. 39, 1860, 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. II ; text figs. 12, 17-19, 21, 22, 24, 29, 30.) 



Illustrated: Walcott, Ann. Lye. Nat. Hist. New York, vol. II, 1875, pi- ; 3ist Ann. Kept. New York 

 State Mus. Nat. Hist, 1879, pi. i, fig. 3; Bull. Mus. Comp. Zool., Harvard Coll., vol. 8, 1881, pi. i, figs. 

 1-5; pi. 2, figs. 1-4, 6-8; pi. 3, figs. 2, 4-7; pi. 4, figs, i, 2, 4-6, 8; pi. 6, fig. 3; Smithson. Misc. Coll., vol. 67, 



