OLENELLUS AND OTHER GENERA OF MESONACID.T: 259 



lobes with their spinose extensions. On large specimens an elongate 

 node occurs at the posterior center of each segment ; it is not known 

 if the posterior eleven segments had median spines of the type occur- 

 ring' on EUiptoccpliala asapJioidcs [pi. 24, fig. i]. The pleural lobes 

 of the anterioi* seventeen seg'ments gradually become shorter until 

 at the seventeenth there is only a trace of the lobe and its median 

 furrow ; each pleura has a broad, strong furrow that is nearly the 

 full width of the segment at its union with the axial lobe, from 

 whence it narrows gradnall\' to the base of the spinose extension 

 of the pleura ; the latter are elongate and gently curved backward 

 near the cephalon ; from whence they increase in length and curva- 

 ture to the seventeenth segment, where their length and backward 

 curvature are so great that they extend a considerable distance back 

 of the posterior eleven segments and pygidium ; the posterior eleven 

 segments of the axial lobe have a backward extending lateral spine 

 attached directly to them [pi. 23, fig. 4] without any intervening 

 grooved pleural lobe. 



Pygidium apparently a continuation of the axial lobe without 

 pleural lobes or spines ; it is small and, as far as can be determined 

 from compressed specimens, it is a simple plate of about ecjual length 

 and breadth that narrows toward the posterior margin. 



Surface minutely granular and with very narrow raised lines 

 or ridges that unite to form an irregular network over glabella and 

 the central portions of the thoracic segments. On the broad cheeks 

 the ridges radiate more or less irregularly from the base of the eye 

 toward the margins of the cephalon. 



Dimensions. — A dorsal shield 41 mm. in length that is flattened 

 in arenaceous shale has the following dimensions. (Two thoracic 

 segments are crowded up beneath the cephalon) : 



Cephalon: mm. 



Length 13-25 



Width at base 31-5 



Length of eye lobe 5- 



Length of glabella 9- 



Width of glabella at base 7- 



Width of glabella at front 4- 



Thora.v: 



Length 26. 



Width at anterior segment, including spinose extension of the 



pleurae 33 



Width at seventeenth segment, including spinose extension 30. 



Width at twenty-eighth segment, including spinose extension. ... 24. 



