V T 6 89. TI ] PROCEEDINGS OF THE NATIONAL MUSEUM. 43 



arches backward from side to side, although very shallow at the center, 

 and it leaves a prominent ridge on each side between it and the pos- 

 terior marginal sulcus; the anterior grooves are short and scarcely 

 more than pits just back of the main body of the hypostoma. Tins 

 hypostoma differs from that of 0. (/If.) Ijerulji and 0. (M.) asaphoides in 

 being narrower anteriorly, more elongate, and with a smooth instead 

 of spinose posterior margin. 



Thorax with eighteen segments.* Axial lobe convex; the center of 

 each segment bears a short, strong, curved spine, the base of which 

 reaches longitudinally across the segment. Pleural lobes flattened, 

 about three-fifths of the distance from the axial lobe to the outer edge, 

 and then gently curving to the ends of the remaining falcate portion of 

 the pleuraB. The narrow, median pleural grooves extend outward to 

 the beginning of the curvature of the broad falcate extremity of the 

 pleura. Pygidium small, transverse, almost quadrangular in outline. 



None of the examples show the details of structure with sufficient 

 clearness. to describe them. 



The surface of the head and thoracic segments is ornamented with 

 the peculiar, inosculating, fine, raised fretwork that, as far as known, 

 is confined to the genus Olenellus. 



Dimensions. — 0. broggeri and 0. thompsonl are the two largest species 

 cf the genus yet described. Fragments of 0. broggeri now before me 

 indicate a length of 24 centimeters. One head has a length of S centi- 

 meters. A bed of greenish argillaceous shale G inches in thickness is 

 almost entirely formed of fragments of large shells. 



The associated faumi includes some well-known Olenellus fauna spe- 

 cies and others not heretofore described. As known now it embraces 

 fourteen genera, twenty-three species, and six varieties. 



Formation and localities. — Lower Cambrian. The best specimens 

 were secured in a reddish-brown argillaceous shale, in a railroad cut, 

 about 1 mile west of Manuel's Brook Bridge, on Conception Bay, New- 

 foundland. It was also found in the limestones beneath Topsail Head 

 and on Brigus Head, on the same bay; at the base of the Manuel's 

 Brook section; where it ranges through 80 feet of strata, and in the 

 decomposed limestone 400 yards west of the brook, in a railroad cut. 

 Stratigraphically its position is 300 feet beneath the Paradoxides zone 

 in the Manuel's Brook section. 



Comparison. — The great occipital spine, small "pleural" spine, broad 

 falcate extension of the pleura?, and short, transverse pygidium dis- 

 tinguish O. broggeri from O. ijerulji and O. mickwitzia of Europe. 

 With the exception of the form of thepleime the same characters sep- 

 arate it from O. asaphoides, 0. Ihompsoni, O. (711.) vermontana, and 0. 

 gilberti. The head of (>. iddingsi is quite distinct. 



* A note made in the field records eighteen segments in the only entire specimen 

 found. Owing to fragile, decomposed rock the pygidium and live segments of this 

 specimen were ground to powder in transporting the large slab which contained it 

 over the rough roads to St. John's. 



