OLENELLUS AND OTHER GENERA OF MESONACID.?£ 339 



three spines are indicated along the postero-lateral margin. It is 

 quite probable that the hypostoma was similar to that represented 

 by fig. 7, pi. 34. 



Comparison zi'ith other species. — The most nearly related species 

 to O. thompsoni [pi. 34, fig. 9] is O. frernonti [pi. 37, figs, i to 7]. 

 The most marked difiference between them is in the larger frontal 

 lobe of the glabella and smaller eye lobe of O. frernonti. The ceph- 

 alon of the latter also has a much greater variation in outline result- 

 ing from the position of the genal angles. From Pccdeiimias transi- 

 tmis [pis. 32 and 34] it differs in having the glabella close to the 

 front marginal border, and in the absence of the rudimentary seg- 

 ments and pygidium posterior to the fifteenth segment, parts which 

 are represented in 0. thompsoni by a strong, long telson. From 0. 

 gilberti [pi. 36] it differs in the cephalon in the same manner as from 

 P. transifans. 



Distribution. — Numerous specimens of the cephalon have been col- 

 lected from the limestone on the north shore of the Straits of Belle 

 Isle, Labrador; on the west coast of Newfoundland at Bonne Bay; 

 along the St. Lawrence Valley from Bic to the Island of Orleans, 

 near Quebec ; on the east side of the Champlain Valley, Franklin 

 County, Vermont ; in central Pennsylvania ; and from thence to 

 central Alabama. 



Dimensions. — The proportions of the various parts of the dorsal 

 shield are clearly shown by the figures on pis. 34 and 35. The 

 larger fragments found indicate a dorsal shield 150 to 160 mm. in 

 length. The average size of adults is from 60 to 100 mm. in length, 

 including terminal telson. 



Formation and Locality. — L^pper part of Lower Cambrian : 

 (25) in the argillaceous shales of Parkers quarry, near Georgia; 

 (25a) 2 miles east of Swanton, on the Donaldson farm; and I noted 

 its presence west of St. Albans, in the outskirts of the city ; in the 

 massive magnesian limestones, west of Parkers quarry, and also 

 about 1.5 miles (2.4km.) east of the hotel at Highgate Springs; 

 all in Franklin County, Vermont. 



This species occurs in the conglomerate limestones of Bic Harbor, 

 Trois Pistoles, St. Simon, and on the Island of Orleans, in the St. 

 Lawrence River, below Quebec, Canada [Walcott, 1886, p. 26]. 



(49d) 3 miles (4.8km.) east of Waynesboro, Franklin County; 

 (49f) Mt. Holly quartzite at Mt. Holly Gap. South Mountain'; 

 (8q) argillaceous shales and limestone, 3 miles (4.8km.) northwest 



^ See also Walcott, 1896, pp. 24-26. 



