ZOOLOGICAL POSITION OF THE TRILOBITES. 39 



most recent. I refer to Walcott's discovery of the 

 fossil Protocaris Marshii, which shows that Apus-\\ke 

 Crustacea were contemporaneous with the very oldest 

 known Trilobites. 



Of the new discoveries which have thrown so much 

 light on the details of organisation of the Trilobites, the first 

 was in August, 1893, when Mr. \V. D. Matthew announced 

 the discovery of specimens of Triarthrus Beckii, showing 

 clear traces of antenna; and of other limbs. 1 The first 

 specimen was discovered by Mr. W. S. Valiant, as long 

 ago as 1884, in the Utica slate, but it was not until 1892 

 that he found other specimens near Rome, New York, 

 which put the matter beyond question. So fine-grained is 

 the matrix, that not only the limbs, but the seta; on the 

 limbs, have left distinct impressions. This discovery, so 

 long w r aited for, naturally riveted the attention of those who 

 had studied the subject at all. It is, however, the name of 

 Dr. C. E. Beecher which in the future will be most in- 

 timately associated with this advance in our knowledge, 

 owing to his patient unravelling of the finer details of 

 structure and organisation, as these could be gathered 

 from a comparison of many specimens. In a series of 

 papers, he has described, in addition to the antennae, the 

 limbs of the thorax, the limbs of the pygidium, the limbs of 

 the head, with the metastoma or under lip of Triarthrus, 

 and, besides these, some of the posterior limbs of Trinucleus, 

 including those of the pygidium. 



I propose here to discuss a few of the more important 

 of these new details, and to show how they reveal the true 

 position of the Trilobites in the genealogical tree of the 

 Crustacea. 



We owe the discovery of the antennae and limbs of Tri- 

 arthrus chiefly to the fact that, in that form, these append- 

 ages were so well developed as to project all round beyond 

 the edge of the dorsal shield. It is a common phenomenon 

 in the animal kingdom to have one member of a group 

 specialised by the great length of the appendages, all 



1 "On the Antennas and other Appendages of Triarthms Beckii" Amer. 

 Journ. Sci., ser. 3, vol. xlvi., p. 121. 



