Raymond & Narraway : Notes on Ordovician Trilobites 247 



to be interpreted as phylogenetically immature or senile phases of the 

 specific type." In the case of Illatnus angusticollis, the specimens 

 showing eight segments are smaller in size than those with nine, and 

 may possibly be immature individuals. 



Locality. — This species has not so far been reported from other 

 than Canadian localities. Billings cited the Island of St. Joseph and 

 the west side of Grant's Island, Lake Huron, and La Petite Chaudiere, 

 Hull, Quebec. The specimens with eight segments here described 

 are from both sides of the Ottawa River at La Petite Chaudiere. Those 

 with nine segments were obtained from the basal beds of the Black 

 River, at Pelton's Quarry, about six miles south of Ottawa, Canada. 



Subgenus Thaleops Conrad. 



Conrad, 1843, Proceedings Academy Natural Sciences, Philadelphia, 

 Vol. I, p. 332. 



Hall, 1847, Paleontology of New York, Vol. I, p. 259. 



Clarke, 1897, Paleontology Minnesota, Vol. Ill, pt. 2, p. 716. 



This subgenus, as defined by Conrad, could hardly be differentiated 

 from Illoznus, and was therefore accorded scant recognition until 

 Clarke brought out the salient characters in describing specimens of 

 Thaleops ovata from Minnesota. These characters, as remarked by 

 Clarke, are the ''peculiar extension of the palpebra and the long, 

 attenuate, and projecting cheeks." 



In the various descriptions of the typical species, much stress has 

 been laid upon the complete isolation of the axial lobe, but this same 

 characteristic is seen in Illcenus angusticollis. Illcenus angusticollis and 

 /. conradi are two forms which almost bridge the gap between the 

 typical Illcenus and Thaleops. 



Thaleops ovata Conrad. 



(Plate LX, figures 11-13; Plate LXI, figures 6, 7.) 



Thaleops ovata Conrad, 1843, Proceedings Academy Natural Sciences, 



Vol. I, p. 332. 

 Thaleops {Illamus') ovatus Hall, 1847, Paleontology New York, Vol. 



I, p. 259, PI. 67, figs. 6a, 6a> 6b. 

 Illoznus ovatus Whitfield, 1882, Geology Wisconsin, Vol. IV, p. 238, 



PL 5, figs. 1, 2. 

 IUceuus herricki Foerste, 1887, Fifteenth Annual Report Geological 



and Natural History Survey of Minnesota, p. 479, fig. 2. 



