ISOTELUS. 33 



across so many segments as to have absolutely prevented any great amount of motion of 

 the segments of the thorax on each other. Enrollment, a common occurrence in Isotclus, 

 would have been absolutely impossible had any such calcified arches been present. 



Walcott, in his study of trilobites in thin section (1881, pp. 192, 206, pi. 2, fig. 9), 

 obtained eleven slices of Isotclus gigas which showed remains of appendages. He figured 

 one of the sections, stating that it "shows the basal joint of a leg and another specimen 

 not illustrated gives evidence that the legs extended out beneath the pygidium, as indicated 

 by their basal joints." 



The second important specimen of an Isotclus with appendages was found by Mr. 

 James Pugh in strata of Richmond age 2 miles north of Oxford, Ohio, and is now in the 

 U. S. National Museum. It was first described by Mickleborough (1883, p. 200, fig. 1-3). 

 In two successive finds, a year apart, the specimen itself and its impression were recov- 

 ered. Since I am redescribing the specimen in this memoir (see p. 35), it only remains to 

 state here that Mickleborough interpreted the structures essentially correctly, though not 

 using the same terminology as that at present adopted. His view that the anterior appen- 

 dages were chelate can not, however, be supported, nor can his idea that the sole appendages 

 of the pygidium were foliaceous branchial organs. 



Walcott (1884, p. 279, fig. i) studied the original specimens and presented a figure 

 which is much more detailed and clear than those of Mickleborough. By further cleaning 

 the specimen he made out altogether twenty-six pairs of appendages. He stated that one 

 of these belonged to the cephalon, nine to the thorax, 1 and the remaining sixteen to the 

 pygidium. He showed that the endopodites of the pygidium were of practically the same 

 form as those on the thorax, and stated that the "leg beneath the thorax of the Ohio 

 trilobite shows seven joints in two instances; the character of the terminal joint is unknown." 

 His figure shows, and he mentions, markings which are interpreted as traces of the fringes 

 of the exopodites. 



In the same year Woodward (1884, p. 162, fig. 1-3) reproduced all of Micklebor- 

 ough's figures, and suggested that the last seven pairs of appendages on the pygidium of 

 Calymene and Isotclus were probably "lamelliform branchiferous appendages, as in Liinuliis 

 and in living Isopoda." 



Professor Beecher published, in 1902, an outline taken from Mickleborough's figure of 

 this specimen, to call attention to certain discontinuous ridges along the axial cavity of the 

 anterior part of the pygidium and posterior end of the thorax. These ridges are well shown 

 in Mickleborough's figure, though not in that of Walcott, and their presence on the speci- 

 men was confirmed by a study by Schuchert, who contributed a diagrammatic cross-section 

 to Beecher's paper (1902, p. 169, pi. 5, figs. 5, 6). Beecher summarized in a paragraph 

 his interpretation of this specimen : 



The club-shaped bodies lying within the axis are the gnathobases attached at the sides of the axis; the 

 curved members extending outward from the gnathobases are the endopodites ; the longitudinal ridges in 

 the ventral membrane between the inner ends of the gnathobases are the buttresses and apodemes of the 

 mesosternites; the slender oblique rod-like bodies shown in the right pleural region in Walcott's figure are 

 portions of the fringes of the exopodites. 



In 1910, Mr. W. C. King of Ottawa, Ontario, found at Britannia, a few miles west of 

 Ottawa, the impression in sandstone of the under surface of a large specimen of Isotelus 

 arenicola, described on a later page (p. 39). 



1 The posterior one of these he believed to have been crowded forward from beneath the pygidium. 



