32 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



of the body, the position of the eye, and even a trace of spines about the pygidium (in 

 the figure) are similar to those of Neolenns, and I would venture the suggestion that 

 Nathorstia transitans is a recently moulted Neolenus serratus, still in the "soft-shelled" con- 

 dition. Even if not a Neolenus, it is probable, from the state of preservation, that it is 

 an animal which had recently cast its shell. 



Walcott describes such fragments of appendages as remain, as follows : 



Head. A portion of what may be an antenna projects from beneath the right anterior margin ; from near 

 the left posterolateral angle a large four-jointed appendage extends backward. I assume that this may be the 

 outer portion of the large posterior appendage (maxilla) of the head. 



Thorax. Traces of several slender-jointed thoracic legs project from beneath the anterior segments and 

 back of these on the right side more or less of six legs have been pushed out from beneath the dorsal shield; 

 these are composed of three or four long slender joints; fragments of the three proximal joints indicate that 

 they are shorter and larger and that they have a fringe of fine setae. Indications of a branchial lobe (gill) are 

 seen in two specimens where the legs are not preserved. This is often the case both among the Merostomata 

 (pi. 29, fig. 3, Malaria) and Trilobita (pi. 24, fig. 2, Ptychoparia). 



Two caudal rami project a little distance beneath the posterior margin of the dorsal shield. 



This latter feature of course suggests Neolenus' The other appendages are too poorly 

 preserved to allow comparison without seeing the specimen. 



The specific name was given "on account of its suggesting a transition between a 

 Merostome-like form, such as Malaria spinifera, and the trilobites." In what respect it 

 is transitional does not appear. 



Formation and locality: Same as that of Neolenus serratus. One nearly complete 

 specimen and a few fragments were found. 



THE APPENDAGES OF ISOTELUS. 

 HISTORICAL. 



The first specimen of Isotclus with appendages was described orally by Billings before 

 the Natural History Society of Montreal in 1864, and in print six years later (1870, p. 

 479, pis. 31, 32). The specimen is described in detail on a later page. Billings recog- 

 nized the remains of eight pairs of legs on the thorax, a pair for each segment, and he 

 inferred from the fact that the appendages projected forward that they were ambulatory 

 rather than natatory organs. He was unable to make out the exact number of the seg- 

 ments in the appendages, but thought each showed at least four or five. 



Having examined the individual sent to London by Billings, Woodward (1870, p. 486, 

 fig, i) reviewed the collection from the American Trenton in the British Museum and 

 found a specimen in the "Black Trenton limestone," from Ottawa, Ontario, in which, along- 

 side the hypostoma, was a jointed appendage, which he described as the "jointed palpus of 

 one of the maxilla;." This has always been considered an authentic "find," but I am in- 

 formed by Doctor Bather that the specimen does not show any real appendage. For 

 further discussion, see under Isotclus gigas. 



In 1871, Billings' specimen was examined by Professors James D. Dana (1871, p. 

 320), A. E. Verrill, and Sydney I. Smith, who agreed that the structures identified by 

 Billings as legs were merely semicalcified arches of the membrane of the ventral surface, 

 which opinion seems to have been adopted by zoologists generally in spite of the fact that 

 the most elementary consideration of the structure of the thorax of a trilobite should have 

 shown its falsity. While the curvature of the thoracic segments was convex forward, that 

 of the supposed ventral arches was convex backward, and the supposed arches extended 



