338 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



OlcncUus tJiompsoni (Hall), Walcott (in part), 1891, Tenth Ann. Rept. 

 U. S. Geol. Snrvey, pi. 82, figs, i and la; pi. 83, figs, i and la (not fig. 

 lb, pi. 83, which is referred in this paper to OlcncUus thompsoni crassi- 

 marginatus). (No text reference. Figures i and la, pi. 82, and i and 

 la, pi. 83, are copied from Walcott, 1886, pi. 23, fig. i ; pi. 17, fig. 9; 

 pi. 22, fig. I ; and pi. 17, fig. 2, respectively. 



Olenclhis thompsoni (Hall), Coi.e, t8o2. Natural Science. Vol. i, fig. i, 

 p. 343. (Gives diagrammatic outline of Walcott's figure, 1886, pi. 17, 

 fig. 2.) 



Olenellus thompsoni (Hall), Frech, 1897, additional plates inserted in 1897 

 in Lethsea geognostica, pt. i, Lethsea Paleozoica, Atlas, pi. la, fig. 7. 

 (Figure 7 is copied from Walcott, 1886, pi. 23, fig. i.) 



Olenellus thompsoni (Hall), Moberg, 1899, Geol. Foren. i Stockholm For- 

 handl., Bd. 21, Ilrifte 4, pp. 314 and 317, pi. 13, fig. 2. (Mentioned at 

 several places in the text. The figure is copied from Walcott, 1886, 

 pi. 17, fig. 2.) 



Olenellus thompsoni (Hall), Lindstkom, 1901, Kongl. Svenska Vet.-Akad. 

 Handlingar, Vol. 34, No. 8, p. 12. (Merely refers to species in dis- 

 cussion of " facial ridge.") 



The adult form of Olenellus thompsoni Hall has been described and 

 illustrated [see Walcott, 1886, p. 167; also 1891, p. 635], but with 

 the discovery of Pccdeiimias transitans [pis. 32, 33, and 34] a limi- 

 tation of the variation in the cephalon has been found that may be 

 of service in identifying the species in the future. The illustration 

 accompanying- the original descriptions of Olenns thompsoni [Hall, 

 1859, p. 60, fig. i] shows a cephalon with strong marginal rim, 

 elongate eyes, and with anterior lobe of the glabella terminating in 

 front at the marginal border, as in our fig. 9, pi. 34. I find in the 

 collections from the locality at Parkers quarry, where the specimen 

 described and illustrated by Hall came from, a number of finely 

 preserved specimens in which the anterior lobe touches the anterior 

 border, as in fig. 9, pi. 34, and in the illustration by Hall. On the 

 cephalon of Pcedeumias transitans the glabella is separated from the 

 marginal border by a distinct space [pis. 32, 33, and 34], except in 

 fig. I. pi. 33, where the glabella is crushed down nearly to the border. 

 This distinction between the cephalon of 0. thompsoni and Pcedeu- 

 mias transitans is found in specimens from Vermont, Pennsylvania, 

 Tennessee, and Alabama. It is quite probable that the separated 

 cephalons of Mesonacis vermontana [pi. 26, fig. i] may be taken for 

 those of 0. thompsoni when the thorax is broken away, but this is 

 not of serious importance as the two species are associated both in 

 Vermont and Pennsylvania. 



The hypostoma of the adult is very rarely preserved at the Georgia, 

 Vermont, localities. In one example [pi. 35, fig. 3] the bases of 



