STRUCTURE AND APPENDAGES OF TRINUCLEUS 221 



Ampyx) are based upon the length of the glabellar spine, and 

 the possession of five or six free thoracic segments. Similar 

 characters in Trinucleus are not considered as worthy of such 

 marked distinction. 



In 1847 Salter * illustrated and described an eye-tubercle on 

 each cheek of Trinucleus^ from which there was a raised line 

 extending obliquely upward to a punctum or spot on each 

 side of the glabella. He considered this line as a discontinu- 

 ous facial suture, but the true suture was afterward correctly 

 determined by Barrande, f and in well-preserved specimens 

 may be easily observed extending around the entire frontal 

 and lateral border of the head, and including the genal 

 spines. The "ej-e-line " was further recognized by McCoy, % 

 and made one of the bases for a division of the genus into 

 two sections or genera, — Trinucleus proper and Tetraspis. 

 These divisions were accepted by Salter, but later were thor- 

 oughly discussed, and rejected by Barrande {I. c, p. 617), 

 upon valid grounds. Nicholson and Etheridge, § in 1879, 

 reviewed these facts at some length, and gave original figures 

 illustrating the ocular tubercle and eye-line. They also 

 agree with Barrande in recognizing them as clearly adoles- 

 cent characters. 



The justice of these conclusions is substantiated, and addi- 

 tional results are reached, from the study of a series of 

 Trinucleus concentricus Eaton, found associated with Triar- 

 thrus Becki Green, in the Utica slate, near Rome, New York. 

 The remarkable preservation of the fossils at this locality 

 has already afforded a means of determining all the principal 

 details of the ventral structure of the trilobite genus Triar- 

 thrus, and there is now distinct evidence as to the nature of 

 the appendages in another type, — Trirmcleus, as well as to 

 the probable significance of the so-called " eye-tubercle." 



* On the structure of Trinucleus, with Remarks on the Species. Quar. Jour. 

 Geol. Soc, III, 251-254. 



t Syst. Sil. Boheme, I, 1852. 



t Ann. Mag. Nat. Hist., 2d Series, IV, 1849. 



§ Monograph of the Silurian Fossils of the Girvan District in Ayrshire, fasc. 

 ii, 1879. 



