4^ THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



Spiral Branchice. 



It is now necessary to inquire if the thin sections can not be interpreted on the basis 

 of trilobites with the same organs as Triarthrus. The interpretation of the structures seen 

 in these translucent slices is exceedingly difficult, and Doctor Walcott deserves the utmost 

 praise for the acumen with which he drew his deductions. Even with the present knowl- 

 edge of Triarthrus, Isotelus, and Neolenus as a guide, I do not think it is safe to speak 

 dogmatically about what one sees in them. 



Walcott has summarized his results in his restoration of the appendages of Calymcne 

 (1918, pi. 33). The coxopodite supports a slender six-jointed endopodite as in Triarthrus, 

 dorsal to which is a short setiferous epipodite which differs from the exopodite of Triar- 

 thrus, in being less long, unsegmented, and in having shorter setae. Arising from the same 

 part of the coxopodite with this epipodite is the bifurcate spiral branchia which has not been 

 seen in this form in other trilobites. The evidence on which the existence of this organ is 

 postulated consists of a series of sections across the thorax, the best of them figured by Wal- 

 cott m his plates 2 and 3 (1881) and plate 27 (1918). 



The specimens sliced were all partially or quite enrolled, and in that position one would 

 expect to find the appendages so displaced that it would be only rarely that a section would 

 be cut, either by chance or design, in such a direction as to show any considerable part of 

 any one appendage. This expectation has proved true in regard to the endopodites, the 

 sections rarely showing more than two or three consecutive segments. Sections like those 

 shown in figures I and 2 in plate 2 (1881) seem to be unique. On the other hand, there are 

 numerous slices showing the so-called spiral branchiae. They show for the most part as 

 a succession of rectangular to kidney-shaped spots of clear calcite. 1 Usually these clear spots 

 are isolated, not confluent, but in a small number of specimens, perhaps three or four, the 

 spots are connected in such a way as to show a zig-zag band which suggests a spiral. Such 

 an explanation is of course entirely reasonable, but it would be surprising if so slender a 

 spiral should be cut in such a way as to exhibit the large series of successive turns shown 

 in many of these thin sections. Continuous sections of such organs should be no more 

 common than continuous sections of endopodites. 



One of the arguments against the interpretation of these series of spots as sections 

 across spiral arms is that of probabilities. It is known from flattened specimens that Neo- 

 Icnus, Kootenia, Ptychoparia, Triarthrus, and Cryptolithus all have a single type of exopo- 

 dite, consisting of a simple setiferous shaft. All these genera have been examined in a 

 way that permits no doubt about the structure, and no trace of spiral arms has been de- 

 tected. On the other hand, Walcott found spiral arms in three unrelated genera, Calym- 

 enc, Ceraurus, and Acidaspis, all of the trilobites in which he found exopodites by the 

 method of sectioning. What are the probabilities that genera of three different families, 

 studied by means of sections, should agree in having a type of exopodite different from 

 that of the five genera about whose interpretation there can be no doubt? 



Another argument against the interpretation of the sections as spirals is that in any 

 one line the individual spots are of roughly uniform size. This means of course that the 

 spiral has been cut by a plane parallel to the tangent plane. This might happen once, just 

 as once Doctor Walcott cut all six segments of a single endopodite, but that it should happen 



1 In looking at Walcott's figures of 1881, it should be remembered that the dark portions of the figures 

 are clear calcite in the specimens, while the light part is the more or less opaque matrix. 



