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 setae 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 setae of several epipodites appear to have been cut across 

 so as to give the effect of long rows of setx. The same condition occurs in specimens of Marrella when 

 the setae of several exopodites are matted against each other. 



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 

 setae in longitudinal section. 

 Specimen 80. X 4- 



This is certainly an apt comparison, and equally true if Neolcnus, Triarthrns, or Cryp- 

 tolithus were substituted for Marrella. 



Now consider the "epipodites." They are well shown in Calymenc in the specimens 

 illustrated on plate 27, figure n (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 setae,, 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 Calymene 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 setae as a 

 series of overlapping spots. A few fortunately located sections in a more nearly hori- 



