80 ROYAL SOCIETY OF CANADA 



The joints of the thonix are strongly arched ; the ring bears a short, 

 stout median spine, and the {)leura' are genieuhited in tlie middle ; some 

 have a weak spine on the posterior margin at the geniculation. Both 

 the pleura and tlie ring have a deep groove along the middle, the one on 

 the i)leura tai»eriiig gradually (as does the pleura itself) outward. The 

 pleura* are bluntly pointed. 



The pygidium found with this species is small and transversely 

 lenticular ; the raehis is wide and large and has two joints ; these are 

 indicated by a pair of short furrows on the outer third ; the furrow 

 dividing otf the half-ring at the front of the raehis is wide and ratlier deep. 

 The side lobes have an elevated lobe at the outer edge which terminates 

 behind in a short spine, directed backward. 



Sculpture. — This consists of tubercles, which are most distinct on the 

 glabella and prominent parts of the shield ; the outer half of the anterior 

 and lateral margins has distinct, anastomosing raised lines, 



Size. — Head-shield. Length exclusive of the occipital spine, 15 mm. 

 Width of the middle piece at the front, 14 ram. ; at the eye-lobe, 18 mm. ; 

 at the posterior angle, 14 mm. Movable cheek, length including the 

 spine. 20 mm. ; width, mm. Pygidium, width, 6 mm. ; length, 3^ mm. 



Horizon and locality. — Common in the shale (No. 3) above the basal 

 conglomerate (Protolenus Zone.) 



This species comes near M. van Imjeni of the St. John Basin, but is 

 smaller and has wider fixed cheeks ; it also has shorter eye-lobes, and the 

 glabella approaches closer to the anterior marginal fold. 



The interocular spine is not conspicuous, and might be overlooked, 

 but it is found on all head-shields that show the margin perfectly. Ford 

 and Walcott have interpreted it in Olenellus {Ellipsocephala) asaphoides 

 as the point of one of the somites of the head-shield. A corresjwnding 

 aculeation is found on the pleura and the side lobes of the pygidium in 

 M. Walcotti ; the one on the pleura, however, is analogous to the genicu- 

 lation of the posterior margin of the middle piece of the head-shield. 



MiOMACCA ANGIMARGO, n. Sp. (PI. IV., fig. 6.) 



A smaller species than the preceding. Only the middle piece of the 

 head-shield is known. It is strongly arched down at the sides. Anterior 

 margin broadly arched, narrow and upturned. Glabella, broad, cylin- 

 drical, considerably raised above the cheeks; only the posterior pair of 

 furrows are seen, and they only towards the margin. The occipital 

 furrow and the doi-sal furrows are deeply and sharply cut, and the 

 occii)ital ring narrow and upturned. The fixed cheeks are wide in front, 

 and bear a distinct ocular tillet, the eye-lobes are shortened, not reaching 

 back to the middle third of the glabella ; it is about as long as half of 

 the width of the glabella ; the cheek extends somewhat behind the eye- 



