1893,] NEW YORK ACADEMY OF SCIENCES. 241 



we may, perhaps, consider that the structure of Triarlhrmt was 

 that of all the Olenidw, the family to which it belongs ; while 

 many of the later trilobites would come nearer to Cahjmene 

 and Ceratd'ifs in their structure. 



As regards the i)robable affinities of the trilobite as modified 

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

 remarks, except tentatively. The homology with Limidiix seems 

 not to be as close in Triar/Jinis as in the forms studied by Mr. 

 Walcott ; but the characters seem to be of a more comprehen- 

 sive type, approaching the general structure of the other Crus- 

 tacea rather than that of any special form. The presence of 

 antennte need not, one would think, separate them from the 

 rest of the Pci'cilopoda, for a small pair occurs in EiirvplfruK, 

 and the anterior pair of appendages in Limulus are also thought 

 to be modified antennre. The cephalic organs are peculiar, if 

 l^roved to be gills, and though in the solitar}^ si)ecimen showing, 

 them they seem to belong to the head, yet further proof would 

 be desirable, that they are not displaced, thoracic limbs. The 

 fused pygidial flap would be a less important character, as it 

 might easily be induced by change of conditions 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, perhaps, 

 than the later types of trilobites. 



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

 other organs, Avill be found in specimens of trilobites unless 

 they be exceptionally well preserved ; but one cluu'acter, the 

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

 mere accident of preservation in these sjiecimens — may be found 

 to exist in less perfect fossils, and would be a fair indication of 

 the former existence of the antenna; which passed under it. 



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

 Kemp in allowing me the honor of describing these specimens. 

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

 in the subject, which has been very useful to me. 



[Note. — As regards the supposed gills under the head, it would seem^ 

 more probable that they wei'e long, thickly set hairs or fimbrite on a nar- 

 row limb, and served as mouth organs rather than gills, though, perhaps, 

 also assisting in the respiration. — June, 1.S93. 



Transactions N. Y. Acad. Sci. Vol. XII. July 22, 1893. 



