2^22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



in the young- and the adult; and (c) in having an unusually ex- 

 panded pleural lobe to the third thoracic segment. A comparison 

 of the young cephalons, as outlined on pi. 37, with those of 0. gilberti, 

 illustrated on pi. 36, shows some of the differences between the two 

 species. 



The variations in the cephalon of 0. frcmonti have been described 

 [Walcott, 1886, pp. 173-178], and reference is also made to them 

 in the introduction to this paper [p. 237]. 



The species that is most nearly related appears to be O. thcmpsoni 

 [pi. 34], but we find that the latter differs from 0. fremonti [pi. 2)7^ 

 in having: (a) a space between the glabella and the marginil rim; 

 (b) a less expanded frontal glabellar lobe and longer palpebral 

 lobes; (c) 0. frcmonti also has a peculiarly expanded pleural lobe 

 of the third segment of the thorax [pi. 37, figs. 6, 6a]. 



The same differences exist in relation to 0. lapzvorfJii [pi. 39]. 

 It differs from O. logani [pi. 41] in details mentioned under that 

 species. 



Olcnclhis frcvwnti is found associated with 0. gilberti in Nevada 

 at locality No. 30; the two species also occur at locality ip, but not 

 in same layer of rock. Comparisons have been made above with 0. 

 gilberti. 



The hypostoma is very rarely preserved. It is much like that of 

 0. gilberti [pi. 36. fig. 5] in having a denticulated posterior margin 

 [pi. 37, figs. 21, 22], and both are much like the hypostoma of 

 Pcrdeumias traiisitans [pi. 34, figs. 6 and 7]. 



The outer surface is similar to that of O. gilberti and other species 

 of the genus. It is beautifully shown by figs. 4 and 5. pi. t,/. 



Dimensions. — The largest specimen of the cephalon in the col- 

 lection has a length of 50 mm., width about 80 mm. This would 

 give an entire dorsal shield, exclusive of the long telson, a length 

 of about 115 mm., which indicates that 0. freniojiti was one of the 

 largest of the Mesonacidse. 



Formation and Locality. — Lower Cambrian: (30) arenaceous 

 shales of the Pioche formation, west slope of Highland Range at 

 edge of desert, 8 miles ( 12.8 km.) north of Bennetts Spring, and 

 about 8 miles (12.8 km.) west of Pioche, Lincoln County; (3i3g) 

 thin-bedded limestones interbedded in shales above a massive series 

 of sandstones, in the Groom Mining District, at the south end of 

 the Timpahute Range, near the line between Nye and Lincoln 

 counties ; (52) arenaceous shales above the Prospect Mountain sand- 

 stones, summit of Prospect Mountain, Eureka District, Eureka 



