196 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 57 



MOLLISONIA, new genus 



General outline elongate, sides subparallel ; cephalon and pygidium 

 siibequal in size and outline, and indistinctly lobate as now known. 

 Eyes unknown. Thorax with seven simple transverse segments 

 that may or may not be divided into axial and lateral longitudinal 

 lobes by slight depressions on the line of each third of the segment. 

 Surface finely granular. 



Genotype, — Mollisonia symmetrica, new species. 



Stratigraphic range. — Middle Cambrian : Burgess shale member 

 of the Stephen formation, British Columbia. 



The generic name is derived from Mollison, the name of a moun- 

 tain southwest of Field on the Canadian Pacific Railway, British 

 Columbia, Canada. 



Observations. — In its subequal cephalon and pygidium this genus 

 resembles Agnostus and Microdiscus, but it differs in the absence of 

 distinct lobes and the greater number of thoracic segments. The 

 almost unsegmented cephalon and pygidium and few simple thoracic 

 segments are characters of the most highly developed families of the 

 Trilobites, such as Asaphidas and Illaenidse, but in the absence of 

 satisfactory evidence of the presence of eyes on the cephalon further 

 study of better material is needed. The form of the thorax recalls 

 that of Bohemillo stupenda Barrande.'' 



MOLLISONIA SYMMETRICA, new species 



Plate 24, fig. 3 



General outline elongate, length about three times the width ; sides 

 subparallel ; cephalon and pygidium nearly equal in size and contour ; 

 thorax with seven segments. Test thin with a minutely granular 

 surface. 



Cephalon a little shorter than its width at the posterior margin ; 

 sides nearly straight and sloping shghtly inward toward the broadly 

 rounded antero-lateral angles and front margin. The presence of 

 eyes is very doubtfully suggested by small, faint, crescentiform de- 

 pressions about 2 mm. from the antero-lateral margin. A raised line 

 that possibly may be the facial suture extends back to the margin 

 midway of the length of the cephalon ; in front of the possible eye 

 lobe it appears to pass across the front parallel to the margin and 

 about 1.5 mm. from it. There are faint traces of transverse furrows 

 indicating the presence of about five transverse lobes on the central 

 portion of the cephalon. 



^ Systeme Silurien du Centre de la Boheme, Vol. I, Suppl, 1872, pi. 14, figs. 

 30 and 32. 



