5O THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



zontal plane might cut the setae and occasionally the shaft of one or more exopodites in 

 the longitudinal plane, and the resulting effect would produce the so-called "epipodites." 

 A careful study has shown that no one of these epipodites is complete, and they do not have 

 the palmate form shown in Walcott's figures. 



And the last and most important argument against the spiral appendages is that cer- 

 tain slices, of both Calymene and Ccraurus, show definitely exopodites of exactly the type 

 found in other trilobites. These are discussed later in the detailed description of the vari- 

 ous slices. 



If these series of spots are interpreted on the basis of the known structure of Triarthrus, 

 they are of course a series of sections through the seta? of the exopodites. It will be shown 

 in Part IV that these setae are not circular in section, but flattened, in Cryptolithus even 

 blade-like, and that they overlap one another. A section across them would give the same 

 general appearance as, for instance, that shown in figures 4, 6, 9, and 10 of Walcott's plate 

 3 (1881). 



When both endopodites and the "spiral branchiae" are present in the same section 

 (pi. I, fig. 4; pi. 2, figs, i, 2), the "spiral branchiae" are dorsal to the endopodites, as the 

 setae of the exopodites would be expected to be. The specimens which show the clear 

 spots connected, and which suggest a spiral (pi. 3, fig. 5), may seem at first sight to bear evi- 

 dence against this interpretation, but one has only to think of the effect of cutting a sec- 

 tion along the edge where the setae are attached to the shaft of the exopodite of Triar- 

 thrus to see that such a zigzag effect is entirely possible. One would expect to cut just 

 this position only rarely, and, in fact, the zigzags are seen in only three or four sections. 

 The bifurcation of the basal segment of the "spiral branchiae" (pi. 3, fig. 10, 1881) is 

 probably more apparent than real, if indeed these basal segments have anything to do with 

 the succeeding one. 



A second peculiarity of Calymene, shown in Walcott's restoration, is the great enlarge- 

 ment of the coxopodites and of the distal segments of the endopodites of the fifth pair of 

 appendages of the cephalon. This is based on the sections of plate 3, figures 6, 7, 8, 9, 

 10 (1881). After a study of the specimens I regret to find myself still unconvinced that 

 the posterior cephalic appendages were any larger than those in front. 



Ventral Membrane. 



The most striking value of the thin sections of Ccraurus and Calymene, and therein 

 they have a great superiority over all the other forms so far investigated, is that they show 

 the extent of the body cavity and the position, though not- the substance, of the ventral 

 membrane. Transverse sections through Ccraurus (Walcott's pi. i, figs. 1-5; pi. 2, figs, 

 i, 3, 1881) and Calymene (pi. 3, figs. 9, 10, 1881) show that the body cavity was 

 almost entirely confined to the axial lobe. The longitudinal sections of Ccraurus (pi. 2, 

 figs. 6, 8; pi. 4, fig. 8) and of Calymene (pi. 2, figs. 5, 7; pi. 5, figs. 1-4) show that the 

 ventral membrane was exceedingly thin and was wrinkled transversely when the shell was 

 enrolled. 



The specimens of figures 1-3, plate 5 (1881) show the form of the ventral membrane 

 more distinctly than any of the others. The section of figure i was cut just inside the 

 dorsal furrow on the right side, and figure 2, which is on the opposite side of the same 

 slice, is almost exactly on the median line. Figure 3 shows a section just inside the left dor- 

 sal furrow. Section 2 did not cut any of the appendages, and the ventral membrane is 



