CERAURUS. 59 



the coxopoclites of two pairs of appendages, one lying nearer the median cavity than the 

 other. It is extremely difficult to visualize the interpretation of such sections. 



Thoracic Appendages. 



A transverse section through a thoracic segment (No. 128, our fig. 17) shows the re- 

 lation of coxopodite to appendifer to be the same as in Calymene, the upper side of the 

 coxopodite having a notch a little outward from the middle. After seeing that specimen, 

 it is possible to understand slice No. 168, which shows longitudinal sections through a num- 

 ber of coxopodites of the thorax, with fragments of both exopodites and endopodites artic- 

 ulated at the distal ends. These and longitudinal vertical sections like No. 18 (pi. 2, fig. 

 8, 1881) show that the endobases taper inward, and the general uniformity in width in 

 sections taken at various angles indicates that the coxopodites were not greatly flattened. 



A unique slice (No. in, pi. 2, fig. 2, 1881 ; pi. 27, fig. i, 1918; our fig. 18) shows a 

 nearly complete thoracic endopodite, and above it a part of the proximal end of the exopo- 

 dite of the same segment. When one considers that out of over two thousand sections only 

 this one shows the six successive segments of an endopodite, one realizes how futile it is 

 to expect that dozens of the equally slender "spirals" should be cut so as to show prac- 

 tically all their turns. 



This endopodite is slender, all the segments have nearly the same length and diameter, 

 though there is a slight taper outward, each segment is expanded distally for the articula- 

 tion of the next, and there are small spines on the distal ends of some of them. There is 

 probably a terminal spine present, though it is neither so long nor so plainly visible as in 

 Walcott's photograph. 



The exopodite on this same specimen was evidently cut diagonally across near the setif- 

 erous edge, showing a section through the shaft and the bases of seven setae (fig. 18). This 

 section is so exactly what would be obtained by cutting similarly an exopodite of either 

 Neolenus or Triarthrus that it should in itself dispose of the "spiral-exopodite" theory. 



Several sections have already been illustrated showing sections across the setae of the 

 exopodites (pi. 3, figs. 4-6, 1881 ; pi. 27, figs. 3, 4, 9, 1918), and similar sections are not 

 uncommon. Only a very few, however, show sections in the plane of the exopodite. If 

 only No. in, described above, were known, it would be inferred that the exopodite had a 

 slender shaft as in Calymene, but another good slice, No. 80 (fig. 12, ante) shows that the 

 blade was rather broad, though not so broad as in Neolenus. The other specimen is No. 22, 

 which has already been discussed. The thoracic exopodite of this specimen has been very 

 incorrectly figured by Walcott, as it shows no such palmate shaft as he has indicated, but a 

 long blade-like one is outlined, though its entire width is not actually shown. 



Pygidial Appendages. 



Sections 14 and 18 (pi. 2, figs. 4, 8, 1881) prove the presence under the pygidium of 

 three pairs of appendages, the coxopodites and fragments of endopodites of which are shown. 

 Nothing is known of the exopodites. 



Relation of Hypostoma to Cephalon. 



In Ceraurus the body portion and posterior end of the hypostoma are roughly oval, 

 about as wide as the glabella at its broadest part, and the posterior edge extends back to 



