88 

 KOTES ON THIARTHRUS SPINOSUS, BILLINGS. 



HENRY M. AMI,B.A. 



.Recent and more detailed observations have brought out new and 

 remarkable features as belonging to this very interesting little trilobite. 



The species, as originally described by Billings, is said to possess 

 four spines, thus described by him in the Report of Progress of the 

 Geological Survey of Canada for the years 1853-56, page 340 : 



" One of these springs from the centre of the neck- 

 segment and extends backwards to the third or fourth 

 segment of the body; a second proceeds from the centre of/ 

 the eighth segment of the axis of the thorax and projects 

 back beyond the apex of the pygidium. Two others, from 

 posterior angles of the head, extend as far as the points of 

 the seventh or eighth pair of pleura. The sjnnes are all 



slender, apparently cylindrical, and about one-fifth of a In^e'^^g^p^j^^^g^^g^" * 



T , )> (after Billings.) 



m diameter. 



These salient characters separate this species very readily from 

 any of the other two species described by Billings, viz.: — T. glaher and 

 T. Fischeri ; also from T. Canadensis, Smith, described in the Canadian 

 Journal for 1861, page 275, and from the common, T. Becki, Greene, 

 in his monograph, p. 87. 



In a paper read before the Ottawa Field Naturalists' Club during 

 the winter of 1881-82, on "The Utica Slate Formation," the writer 

 had occasion to notice, in regard to the same species, that whilst the 

 majority of specimens found indicated in the presence of a spine on the 

 e'ujhih thoracic segment, still, in a few specimens he was able to observe 

 another spine on the ninth tlioracic segment. It was this fact which 

 led him to say : " for the present we are satisfied with stating that it 

 {T. spinosus) possessed at least more than four spines." 



Now, the most perfect specimen yet procured from the deposits of 

 the Utica at^Cummings' Bridge, Gloucester, Ont., shows clearly that, 

 not only did the species in question possess "more than four spines," 

 but that the eighth, the ninth, and likewise the tenth thoracic segments 

 of the one individual have each a spine proceeding from the central 

 portion of their axis, which three spines, along with that on the occipital 



