140 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



be best explained as a trilobite which grew to the adult state without losing its protaspian 

 form. It was found in the Middle Cambrian of British Columbia. 



Even if Naraoia should eventually prove to possess characteristics which preclude the 

 possibility of its being a primitive trilobite, it at least represents what I should expect a 

 pre-Cambrian trilobite to look like. What the ancestry of the nektonic primitive trilobite 

 may have been is not yet clear, but all the evidence from the morphology of cephalon, pygid- 

 ium, and appendages indicates that it was a descendant of a swimming and not a crawling 

 organism. 



Since the above was written, the Museum of Comparative Zoology has purchased a 

 specimen of this species obtained from the original locality. The shields are sub-equal, the 

 posterior one slightly the larger, and the axial lobes are definitely outlined on both. The 

 glabella is about one third the total width, nearly parallel-sided, somewhat pointed at the 

 front. There are no traces of glabellar furrows. The axial lobe of the pygidium is also 

 about one third the total width, extends nearly to the posterior margin, and has a rounded 

 posterior end. The measurements are as follows : Length, 33 mm. ; length of cephalon, 



16 mm., width, 15 mm.; length of glabella, 11.5 mm., width, 5.5 mm.; length of pygidium, 



17 mm., width, 15 mm.; length of axial lobe, 14 mm., width, 5.5 mm. 



The species is decidedly Agnostus-like in both cephalon and pygidium, and were it not 

 so large, might be taken for the young of such a trilobite. The pointed glabella is com- 

 parable to the axial lobes of the so called pygidia of the young of Condylopyge rex and Pero- 

 nopsis integer (Barrande, Syst. Sil., vol,. i, pi. 49). 



THE ANCESTOR OF THE TRILOBITES, AND THE DESCENT OF THE ARTHROPODA. 



The "annelid" theory of the origin of the Crustacea and therefore of the trilobites, 

 originating with Hatschek (1877) an d so a ^Y championed by Bernard (1892), has now 

 been a fundamental working hypothesis for some years, and has had a profound influence in 

 shaping thought about trilobites. This hypothesis has, however, its weak points, the prin- 

 cipal one being its -total inhibition of the workings of that great talisman of the palaeontol- 

 ogist, the law of recapitulation. Its acceptance has forced the zoologist to look upon the 

 nauplius as a specially adapted larva, and has caused more than one forced explanation of 

 the protaspis of the trilobite. When so keen a student as Caiman says that the nauplius 

 must point in some way to the ancestor of the Crustacea (1909, p. 26), it is time to reex- 

 amine some of the fundamentals. This has been done in the preceding pages and evidence 

 adduced to show that the primitive features of a trilobite indicate a swimming animal, and 

 that the adaptations are those which enabled it to assume a crawling mode of existence. 

 It has also been pointed out that in Naraoia there is preserved down to Middle Cambrian 

 times an animal like that to which ontogeny points as a possible ancestor of the trilobites. 

 Naraoia is not the simplest conceivable animal of its own type, however, for it has built 

 up a pygidium of fourteen or fifteen somites. One would expect to find in Proterozoic sedi- 

 ments remains of similar animals with pygidia composed of only one or two somites, with 

 five pairs of appendages on the cephalon, one or two pairs on the pygidium, a ventral mouth, 

 and a short hypostoma. Anything simpler than this could not, in my opinion, be classed as 

 a trilobite. 



What the ancestor of this animal was is mere surmise. It probably had no test, and 

 it may be noted in this connection that Naraoia had a very thin shell, as shown by its 



