^^Q ARTHROPODA phylüm vii 



^tudwitrÄr^^^^^ to its termination in the anal open.ng at the 



^•^'Z; :fth1 :tÄ— 'l^Sowledge of Mobite s«. have 

 con.e ffom the study o£ numerous very perfectly preserved specimens of 



Triarthrus becki Green. 



Fio. 1342. 



Utica Sliale (Ordovician) ; (Rome, New York. 

 2/1 (after Beecher). 



A, Dorsal, and B, Ventral aspect, 



Triarthrus hecki Green, from the Utica Shale (Ordovician), near Rome, New 

 York. Undoubted antennae in this form were discovered by Valiant, and first 

 announced by Matthew in 1893. Subsequently a series of papers was 

 published by Beecher on the detailed structure of this Trilobite, which is now 

 the best known of any species, and necessarily forms the basis of much of the 

 following summary of ventral organs. 



In the median line anteriorly, there is first the hypostoma or upper lip, at 

 the end of which, and opening obliquely back ward, is the mouth (Walcott, in 

 Calymene). In Triarthrus the lower lip, or metastoma, is a convex arcuate 

 plate, just posterior to the extremity of the hypostoma. At the angles on 

 either side are two small elevations, or läppe ts. 



