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8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



the head a trapezoidal shape, making it shorter and wider. Again the 

 facial suture shows plainly posterior to the eye but does not appear in 

 front of it. The glabella of M. bristolcnsis extends farther forward 

 so that it apparently overhangs the rim. It dififers also from M. 

 frcmonti in having a greater constriction where the dorsal furrow is 

 bent inward opposite the third pair of glabellar furrows, thus giving 

 the glabella somewhat of an hour-glass shape. The occipital ring 

 is wider than in M. frcmonti, and the furrow, while also interrupted, 

 dififers in having the shallowing inner ends turn sharply backward 

 before dying out completely. The third pair of glabellar furrows also 

 does this, and the two lateral portions are parallel to the two portions 

 of the occipital furrow. The second pair of glabellar furrows is 

 represented by a continuous line, only slightly curved back in the 

 center — less than in M. fremonti. The first pair of furrows is similar 

 in direction and depth in both species. 



The rim of M. bristolcnsis appears to narrow toward the center of 

 the head, owing to the more forward extension of the glabella. It is 

 striated as usual and the intramarginal suture running along the inner 

 edge of the marginal plate is clearly marked. This suture occupies the 

 usual relative position on the greatly shortened rim and across the 

 genal angle. It continues sub-parallel to the posterior edge of the free 

 cheek, the outer edge of which in this case occupies a vertical posi- 

 tion, constituting the lateral margin of the cephalon, and terminates in 

 the lower corners of the cephalon where the facial suture reaches the 

 margin. 



The palpebral lobes are relatively further forward, shorter, and 

 perhaps a little more curved than in M. frcmonti. 



Mcsonacis bristolcnsis has the usual striated surface, but the striae 

 appear a little stronger than in the other species. 



MESONACIS INSOLENS, new species 

 Plate 2, figs. 1-4 



More than 20 cephala of this species occur in the collections, and 

 again none shows any variation in the point of origin and direction of 

 the advanced genal spines. A poorly preserved, almost entire specimen 

 indicates some of the characters of the thorax, which also will not 

 fit any of the described forms with similarly advanced genal angles. 



Compared with M. bristolcnsis, the most similar species in this 

 fauna, several differences beside that of the position of the genal spine 

 are readily noticeable. The shape of the cephalon is normal, being 

 quite like that of M. frcmonti. The posterior portion of the facial 



