CALYMENE AND CERAURUS. 49 
repeatedly is highly improbable. Moreover, there is a limit to the diameter of the section 
which may be made from these slender spirals. Most of the spots have one diameter about 
one half greater than the other, but others are from three to six times as long as wide. 
These last could obviously be cut only from a very large spiral, and they are therefore 
interpreted by Walcott as seta? of epipodites. Yet all gradations are found among the sec- 
tions, from the long set« to the short dots. (See pi. 27, 1918. ) In referring to one slice, 
Walcott says (1918, p. 152) : 
In the latter figure and in figure 13, plate 27, the seta of several epipodites appear to have been cut across 
so as to give the effect of long rows of setae. The same condition occurs in specimens of Marrella when 
the setae of several exopodites are matted against each other. 
^''SfW 
Fig. 12. — A slice of Ceraurus 
pleurexanthemus in which the 
exopodite happened to be cut 
in such a way as to show a part 
of the shaft and some of the 
setse in longitudinal section. 
Specimen 80. X 4- 
This is certainly an apt comparison, and equally true if Neolcnus, Triarthrus, or Cryp- 
tolithus were substituted for Marrella. 
Now consider the "epipodites." They are well shown in Calymcnc in the specimens 
illustrated on plate 27, figure 11 (1918), and plate 3, figure 3 (1881), and less clearly in 
one or two others. Slices 22 (pi. 27, fig. 12, 1918) and 80 (our fig. 12) show what is 
called the same organ in Ceraurus. It will be noted that all of these slices are cut in 
the same way, that is, more or less parallel to the under surface of the head, or, at any rate, 
on a plane parallel to a plane which would be tangent to the axial portion of the coiled shell. 
The sections which show the spirals best are those which are cut by a plane perpendicular to 
the long axis of the body. If one were to attempt to cut an enrolled Triarthrus in such a 
way as to get a section showing the length of the seta?, one would not cut a section per- 
pendicular to the axis of the animal, nor, in fact, would he cut one parallel to the ventral 
plane, but it is obvious that in this latter type of section he would stand a better chance of 
finding a part of the plane of the exopodite coincident with the plane of his section than in 
the former. And that seems to be what has happened in these sections of Calymcnc and 
Ceraurus. If the exopodites were preserved, transverse sections were bound to cut across 
many sets of fringes, and the resultant slice would show transverse sections of the seta? as a 
series of overlapping spots. A few fortunately located sections in a more nearly hori- 
