REPORT OF THE DIRECTOR 1919 



71 



The writer has found this characteristic trilobite of the Canadian 

 Gloucester shale among material from the upper Utica shale at Hol- 

 land Patent, N. Y. These specimens exhibit a series of three suc- 

 cessive spines on the axis of the eighth, ninth and tenth thoracic 

 segments, and show further the genal spines to be inserted in a 

 peculiar way forward of the genal angle, at a point about one-third 

 the length of the head. Moreover the latter do not proceed back- 

 ward but are curved outward at the base and slightly inward dis- 

 tally. Fine niaterial of Triarthrus spinosus in the United 

 States National Museum, indicates by the presence of spine bases 

 that also the nth to 13th thoracic segments bore spines. Finally, 

 also some of the pleurae of the thorax seem to have been produced, 

 at times, into long recurving spines ; this is at least suggested by 

 two such spines, found apparently attached to a fragment of a 

 pleura, lying partly below a specimen. 



Fig. 8 



Figs. 7, 8 Triarthrus spinosus Billings. Fig. 7 : Billings's figure ; 

 fig. 8: laterally compressed specimen (x2), showing dorsal spines (from the 

 Gloucester shale at Ottawa, Canada; original in U. S. National Museum). 



The peculiar lateral spine of the free cheek, the series of long axial 

 spines on the posterior segments, are characters not seen in Ordovi- 

 cian or later trilobites, but known of certain early Cambrian forms, 

 especially of the family Mesonacidae. Also the long nuchal spine, 

 on the occipital ring is very strongly developed in the Mesonacidae, 

 but rarely seen in Ordovician and later trilobites. 



Triarthrus is now the only Ordovician genus of the Cambrian 

 family Olenidae (Olenus barely entering the Ordovician in Europe) 

 and the last of that fam_ily which in its turn appears again as a 

 further development of the Lower Cambrian family Mesonacidae. 

 The characters here emphasized of T. spinosus and which dis- 

 tinguish this last of the Triarthri (the Gloucester shale being of 



