66 Field Museum of Natural History — Geology, Vol. IV. 



five anterior segments terminate at the lateral margins of the thorax, 

 but the seven posterior ones are abruptly bent backward at the lateral 

 thoracic margins and are produced into spines. The spines of the sixth 

 segment extend more than one-third their length beyond the pygidium ; 

 the spines of the other segments are considerably shorter. 



Pygidium small but too poorly preserved in the specimens at hand 

 for a detailed description. 



The dimensions of the type specimens are as follows: 



Length of the holotype along the axis 23.6 mm. Length of body, 

 including pleural spines, 3-1.3 mm.; width at genal angles 19.6 mm.; 

 length of cephalon, including genal spines, 16 mm. ; length of glabella 7.8 

 mm.; width of anterior portion of glabella 4.7 mm.; ditto across lateral 

 lobes, 5.3 mm.; length of thorax 12.8 mm. 



Width of paratype at genal angles 27.8 mm.; length of glabella 10.7 

 mm.; width of posterior portion 8.0 mm.; ditto across lateral lobes, 8.7 

 mm. 



This description is based upon the holotype (Mus. No. P 16631, 

 PI. XVI, fig. 1), a nearly complete specimen, a practically complete 

 cephalon (Mus. No. P 17039, PI. XVI, figs. 2-3), somewhat larger than 

 the holotype, and a detached free cheek with eye (Mus. No. P 16633, 

 PI. XVI, fig. 4). 



Localities and horizon. — The holotype was collected by the writer 

 from the top of the Lower Maquoketa beds at Elgin, the two paratypes 

 from a similar horizon at Bloomfield. More or less complete cranidia are 

 not uncommon at a similar and somewhat lower horizon at Clermont. 



Family CALYMENID^, Brongniart 

 Genus CALYMENB Brongniart 1822. 



"Complete body sub-oval in outline, possessing the power of com- 

 plete enrollment; cephalon sub-crescentiform with thickened margin, 

 genal angles usually rounded; glabella strongly convex, narrowed in 

 front, with three pairs of deep lateral furrows; occipital segment well 

 defined; posterior limb of the facial sutures originating just in front of 

 the genal angles; free cheeks elongate, separate, usually with a free plate 

 between their anterior extremities in front of the glabella; eyes small; 

 hypostoma sub-quadrate, notched. Thorax with thirteen segments; 

 axis strongly convex and bounded by deep axial furrows; pleural lobes 

 wider than the axis, bent down laterally. Pygidium distinctly marked 

 off from the thorax, with six to eleven segments, axis prominent and 

 margin entire." (Weller, Pal. Chicago Area, p. 261.) 



