W, D. Matthew — Antennae of Trilobites. 125 



liar cephalic organs, and the anchylosed flap under the 

 pygidium, would all be important points of difference, and 

 indicate that the Trilobites were quite varied in structure, and 

 probably included several widely differing types. If the 

 classification founded on the characters of the shield is not 

 deceptive, we may perhaps consider that the structure of 

 Triarthrus was that of all the Olenidae, the family to which it 

 belongs ; while many of the later trilobites would come nearer 

 to Calymene and Ceraurus in their structure. 



As regards the probable affinities of the trilobite, as modi- 

 fied by Mr. Valiant's discovery, the writer can scarcely venture 

 any remarks, except tentatively. The homology with Limulus 

 seems not to be as close in Triarthrus as in the forms studied 

 by Mr. Walcott ; but the characters seem to be of a more com- 

 prehensive type, approaching the general structure of the 

 other Crustacea rather than that .of any special form. The 

 presence of antennae need not, one would think, separate 

 them from the Pcecilopoda ; for a small pair occurs in Euryp- 

 terus, and the anterior pair of appendages in Limulus are 

 thought to be modified antennae. The cephalic organs are 

 peculiar, if proved to be gills, and though in the solitary speci- 

 men showing them they seem to belong to the head, yet 

 farther proof would be desirable that they are not displaced 

 thoracic limbs. The fused pygidial flap may be a less import- 

 ant character, as it might be induced by some change of con- 

 ditions of life. 



As regards the habits of Triarthrus, we may conjecture that 

 it usually scuttled through the soft mud which composed the 

 shale in which it is found, on the little pointed walking legs ; 

 but that it had considerable swimming powers, more so per- 

 haps than the later types of trilobites. 



It is hardly to be expected that these antennae, still less the 

 other organs, will be found in specimens of trilobites unless 

 they are exceptionally well preserved. But one character, the 

 arching of the anterior rim at the center of the head — if not a 

 mere accident of pieservation in these specimens — may be 

 found to exist in less perfect fossils, and would be a fair indi- 

 cation of the former existence of antennae which passed under it. 



In conclusion, I wish to acknowledge the kindness of Prof. 

 Kemp in allowing me the privilege of describing these speci 

 mens. I have also to thank Prof. Whitfield for advice and 

 assistance which have been very useful to me in the subject. 



Note. — As regards the supposed gills under the head, it 

 would seem more probable that they were long, thickly set 

 hairs or fimbriae on a narrow limb, and served as mouth organs 

 rather than gills, though perhaps also assisting in the respira- 

 tion. 



Geological Department, Columbia College. 



