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THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 



Anisopyge, the last of the trilobites, stands third on the list of those having great 

 numbers of segments, and in each period there are a few which have considerably more 

 than the average number. It may be of some significance that of these nine genera only 

 Pcedeumias and Anisopyge belong to the Opisthoparia, the great central group, and that five 

 are members of the Proparia, the latest and most specialized order. 



FORM OF THE SIMPLEST PROTASPIS. 



It would naturally be expected that the young of the Cambrian trilobites should be 

 more primitive than the young of species from later formations, and Beecher (1895 C) 

 has shown that this is the case. He had reference, however, chiefly to the eyes, free cheeks, 

 and spines, and by comparison of ontogeny and phylogeny, demonstrated the greater sim- 

 plicity of the protaspis which lacked these organs. It remains to inquire which among the 

 other characteristics are most fundamental. 



Among the trilobites of the Lower Cambrian, no very young have been seen except 

 of Mesonacidae. Of these, the ontogeny of Elliptocephala asaplioides Emmons is best known, 

 thanks to Ford, Walcott, and Beecher, but, as the last-named has pointed out, the actual 

 protaspis or earliest shield has not yet been found. The youngest specimen is the one roughly 

 figured by Beecher (1895 C, p. 175, fig. 6). It lacks the pygidium, but if completed by 

 a line which is the counterpart of the outline of the cephalon, it would have been 0.766 mm. 

 long. The pygidium would have been 0.183 mm. long, or 23 per cent of the whole length. 

 The axial lobe was narrow, of uniform width along the cephalon, showed a neck-ring and 

 four indistinct annulations, but did not reach quite to the anterior end, there being a mar- 

 gin in front of the glabella about o.i mm. wide. The greatest width of the cephalon was 

 0.66 mm., and of the glabella 0.233 mm., or practically 35 per cent of the total width. 

 Other young Elliptocephala up to a length of i mm., and young Pcedeumias, Mcsonacis, and 

 Holmia (see Kiaer, Videnskaps, Skrifter, I Mat.-Naturv. Klasse, 1917, No. 10) show about 

 the same characteristics, but all these have large compound eyes on the dorsal surface and 

 specimens in still younger stages are expected. It may be pointed out, however, that in 

 these specimens the pygidium is proportionately larger than in the adult. Walcott cites one 

 adult 126 mm. long in which the pygidium is 6 mm. long, or between 4 and 5 per cent of 

 the total length, while in the incomplete specimen described above, it was apparently 23 per 



