OLENELLUS AND OTHER GENERA OF MESONACID.E 349 



are strongest at the first, second, and fourth lobes. Palpebral lobes 

 prominent and extending- obliquely outward and backward opposite 

 the posterior half of the first glabellar lobe, and the entire second 

 glabellar lobe ; a strong ridge unites the palpebral lobe on each side 

 with the side of the first glabellar lobe ; the eye rises abruptly from 

 the surface of the cheek opposite the genal spine and is so curved 

 that the visual surface 'commands all parts of the dorsal shield ex- 

 cept directly back of the glabella. I have not been able to find any 

 trace of a furrow connecting the posterior end of the eye with the 

 intergenal spine ; if such existed it would extend along the outside 

 of the longitudinal ridge next to the glabella. The inner slope from 

 the palpebral lobe to the dorsal furrow beside the glabella is quite 

 steep and gives great prominence to the eye. The elongate ridge 

 on each side that rises between the eye and the glabella extends 

 back to where it passes out into the intergenal spine ; this inter- 

 palpebral ridge appears to be formed of the extensions of the gla- 

 bellar lobes in the same manner as a som.ewhat similar ridge on the 

 cephalon of the young of Elliptocephala asaphoides [pi. 25, figs. 9, 

 10]. In one specimen traces of segmentation are preserved on the 

 ridges, and a connecting ridge crosses the dorsal furrow uniting 

 the second and fourth glabellar lobes with the longitudinal ridge. 

 The spaces between the front of the glabella, the eyes, and the longi- 

 tudinal ridge back of the eyes and the marginal rim are very small 

 and of little importance. 



The thorax is largely formed of the axial lobe and spinose exten- 

 sions of the seven thoracic segments (Dr. Peach mentions eight 

 segments, but I can not make out more than seven). Axial lobe con- 

 vex and separated from the pleural lobes by a clearly defined dorsal 

 furrow ; a minute median tubercle or spine occurs near the posterior 

 margin of the segment and there are slight traces of furrows that 

 begin near the anterior center of the segment and extend obliquely 

 outward and backward to the postero-lateral edge, thus repeating 

 the surface structure of the occipital ring. The first three segments 

 are about as wide as the occipital ring, the others gradually narrow 

 toward the pygidium ; pleural lobes narrow ; those of the first seg- 

 ment are shorter than their width, or longitudinally quadrilateral in 

 outline ; those of the second segment are a little longer than those 

 of the first and those of the fourth and fifth are longer than wide ; 

 the pleurae of the third segment are broader than those of the first 

 and second, and are prolonged into a long spine that extends ob- 

 liquely outward and backward at about the same angle as the inter- 



