7. STRUCTURE AND APPENDAGES OF 

 TRINUCLEUS * 



(PLATE X) 



TRINUCLEUS departs so widely from the common type of 

 trilobite form that any contribution of new facts regarding its 

 structure and appendages is a matter of interest. Moreover, 

 this added information will be of assistance in interpreting 

 some peculiar and striking features in the natural group of 

 genera of which Trinucleus is evidently a member. 



For the present it is convenient to consider in this group 

 such forms as Trinucleus, Harpes, Harpides, Dionide, and 

 Ampyx. Most of these have the genal angles extending to 

 or beyond the pygidium, with a broad, finely perforated or 

 punctate margin around the head. They are further char- 

 acterized by the absence or obsolescence of visual organs, 

 while the facial sutures are either peripheral, as in Harpes, 

 or in addition include the genal spines, as in Trinucleus^ 

 Dionide, and Ampyx. Several other genera have been recog- 

 nized as having affinities with those mentioned, but they are 

 imperfectly known, and will be merely noticed here. Har- 

 pina Nova"k, based upon the features of the hypostoma, is 

 probably of only sub-generic value under Harpes. Arraphus 

 Angelin is apparently based upon a specimen of Harpes 

 denuded of the punctate border. Salteria of W. Thompson 

 and Endymionia of Billings, both generally considered as 

 closely related to Dionide, were founded upon too imperfect 

 material to afford decisive data as to their affinities. Ange- 

 lin's sub-genera of Ampyx (Lonchodomus, Raphiophorus, and 



* Amer. Jour. Set. (3), XLIX, 307-311, pi. iii, 1895. 



