OLENELLUS AND OTHER GENERA OF MESONACID^ 2/3 



glabella to the outer rim ; on the outer rim and spines and on the 

 falcate spinous extension of the pleurae the ridges are subparallel 

 to the margins. 



Hypostoma elongate ovate in outline, strongly convex ; anterior 

 margin arched to conform to the outline of the interior margin of 

 the doublure of the cephalon to which it was attached ; posterior 

 margin and side margins to the antennal groove denticulated or with 

 six or more short, blunt projections on each side of the median line ; 

 antennal groove on the lateral margins in front of the thickened 

 round rim and back of the subtriangular anterior lateral extension 

 of the body along the anterior margin. The oval body is outlined 

 posteriorly by a strong furrow on each side that extends obliquely 

 inward and backward from the antennal furrow, and a very shallow 

 transverse furrow that unites the posterior ends of the oblique fur- 

 rows ; a short, shallow, transverse furrow occurs on the space be- 

 between the furrow described and the denticulated posterior margin. 



It is rarely that the denticulated margin can be worked out of 

 the hard limestone matrix. This led Ford [1877, pi. 4, fig. 6] and 

 Walcott [1886, pi. 17, fig. 10] to represent the hypostoma with a 

 smooth posterior margin. A denticulated margin was illustrated but 

 not described by Walcott in 1891, pi. 88, fig. i^. 



Dimensions. — A dorsal shield 126 mm. in length that is flattened 

 in the shale has the following dimensions : 



Cephalon: mm. 



Length 50 



Width at base 98 



Length of eye lobe 24 



Length of glabella 40 



Width of glabella at base 24 



Width of glabella at front 31 



Thorax: 



Length .* 70 



Width at anterior segment, including spinose extension of the 



pleurae 79 



Width at thirteenth segment, including spinose extension 63 



Width at eighteenth segment, including spinose extension 14 



Width of axial lobe, anterior segment 24 



Width of axial lobe, thirteenth segment 13 



Width of axial lobe eighteenth segment " 9 



Width of pleural lobe, anterior segments 20 



Width of pleural lobe, eighteenth segment 3 



Pygidium: 



Length about 6 



Width at front 13 



