OLENELLUS AND OTHER GENERA OF MESONACID^ 327 



fig. 5], in this respect being similar to those of JJmidus [pi. 43, 

 figs. 1-4]. The lenses, as seen in one specimen, are arranged in 

 quincunx order, the rows crossing the visual surface of the eye 

 obliquely between the upper and lower margins. The narrow ridges 

 between the lenses are rounded and have the same exterior appear- 

 ance as the outer surface of the eye of Livnuliis polyphemus. The 

 cheeks rise rather abruptly from the rounded intermarginal furrow 

 and gently arch to the base of the eye and first glabellar lobe. A 

 narrow, elevated line or ridge extends outward from the posterior 

 base of the eye and crosses the posterior border obliquely so as to 

 terminate at the intergenal angle or is continued into a short spine. 

 This ridge follows the line of the facial suture which is probably in 

 a condition of symphysis ; no traces of the facial suture have been 

 observed in front of the eye. 



The only specimens preserving the thorax are flattened in shaly 

 sandstone. The one illustrated has been compressed and distorted 

 [fig. 9], but it shows the general form of the thorax and its seg- 

 ments. Fourteen segments and the base of the telson-like terminal 

 spine can be determined. The axial lobe is convex and about one- 

 half the width of the pleural lobes ; a very short median spine, or 

 sharp, elongate node, occurs at the posterior margin of each segment 

 with its rather strong base reaching nearly half way across the 

 segment ; the pleural portions of each segment extend directly out- 

 ward for a distance about one-half of their length and then curve 

 gradually backward, passing into a slender, spine-like extension: 

 the pleural furrow is broad and of nearly equal width from its inner 

 end out to the geniculation of the pleura where it begins to narrow. 

 The enlargement of the third segment is the same as in Olenellus 

 thompsoni [pi. 35] and Pcudeumias [pi. 34]. The telson is long 

 and slender, and much like that of 0. frcmonti [pi. 37, fig. 7] and 

 Olenellus thompsoni [pi. 34, fig. 9]. 



Surface ornamented with irregular, fine, inosculating ridges that 

 form a very fine network of varying pattern. On the border and 

 cheeks the meshes are small, elongated, and subparallel to the mar- 

 gin : over the glabella the meshes are very fine and the same is true 

 for the surface of the thorax : the interspaces between the ridges 

 appear to be minutely granular. The inner surface of the cheeks 

 is beautifully channelled by irregular canals that radiate from the 

 base of the eye outward toward the intermarginal groove ; the chan- 

 nels often run into each other, and they are frequently united by 

 cross channels. 



