[parks] TRIARTHRUS-CANADENSIS, GLABER, SPIXOSUS 49 



line from the posterior margin to the rear of the eye. All the free 

 cheeks found conform to this latter pattern leading to the conclusion 

 that it is the normal course of the suture. The explanation seems to 

 be that the suture was an overlapping one which would give a different 

 course on the cast of the interior and the mould of the exterior. 



The marginal furrow arises at the point where the axial furrow 

 crosses the line between head and thorax, sweeps outwards and for- 

 wards with increasing width and follows the margin of the head to 

 the front; it becomes much narrower in front of the glabella. 



The palpebral lobe is close to the glabella, long, and but slightly 

 cur\'ed; it is accentuated by a depression internally. 



The free sheek is wide in front as in T. glaher but the genal angle 

 is drawn out into the spine characteristic of the species. In this 

 specimen the spine could not have been more than two or three 

 millimetres long but it is broken on both sides. There is no indication 

 of the median furrow referred to by Smith. 



The occipital ring of the glabella carries a small but distinct 

 tubercle. 



The thorax consists of thirteen rings. The axial part ot each 

 segment is smooth and rounded, but where decorticated a faint 

 crescentric indentation is seen in the matrix. The pleurae are 

 strongly furrowed and there is a secondary short furrow extending 

 outwards from the axial furrow immediately in front of the posterior 

 margin. 



The pygidium is relatively small and seems to have been formed 

 from six primary segments. The axial portion is well defined and the 

 individual rings are narrower and sharper than those of the posterior 

 thoracic segments. The pleural grooves are symmetrical with those 

 of the thorax; the anterior four are well marked but do not reach the 

 margin of the pygidium; the posterior two are ill defined. 



As already stated, many free cheeks were found which un 

 doubtedly belong to this species; they indicate specimens of much 

 greater size and show the groove on the spine referred to by Smith 

 (Figure 3). \'ery similar are the free cheeks from Craiglieth 

 (P'igure 1). In both instances I am of the opinion that no signifi- 

 cance attaches to the groove; it is probably the result of the 

 flattening of a hollow structure and is observed in spines of T. spinosus 

 and other species. 



All the detached cranidia found on the Rouge fall distinctly into 

 two series. Those with a glabella narrowing anteriorly, eyes forward 

 and well in, squarish front, and occipital tubercle are ascribed to 



