NO. 2 CAMBRIAN FOSSILS FROM MOHAVE DESERT RESSER J 



California here illustrated agrees in every respect with that from the 

 type locality — Eureka District, Nevada (Loc. 52), to which the 

 species is now restricted, and also with the Specimens from Resting 

 Springs, Inyo County, California (Loc. 14L). This is the most abun- 

 dant species at Bristol Mountain, being represented by more than 50 

 specimens, of which only two show a portion of the thorax. The speci- 

 mens vary in size from less than one cm. in width of cephalon to more 

 than 12 cm., and the position of the genal spine is exactly the same 

 in all. 



The illustrations show the intramarginal suture that begins on the 

 posterior margin of the cephalon, crosses over the genal angle, and 

 then passes forward just inside the strongly striated rim, separating 

 a marginal plate from the cheeks (see p. 5). This suture, as it passes 

 forward, leaves the exact inner edge of the rim and runs along on it 

 becoming less well defined, but does not reach the outer margin. A 

 second suture appears to be present on the margin, thus forming a 

 second detachable plate that lies under the first. This second plate, 

 to which the hypostoma is probably attached, should, I think, be re- 

 garded as corresponding to the rostrum or epistoma of other trilobites. 

 Just what the upper plate, which carries the upper half of the genal 

 spines, may represent is not clear. Provisionally I shall call this plate 

 the marginal plate. In some specimens the marginal plate is broken 

 away (pi. i, figs. 3, 4) exposing the underlying epistoma; in others 

 both plates have been lost. 



The facial suture, the position of which in the trilobites of this 

 family has been a matter of much discussion, is quite clearly indicated 

 in normal position posterior to and along the eyes, but its course 

 anterior to them is not apparent. 



MESONACIS BRISTOLENSIS, new species 



Plate 2, figs. 5-8 



None of the illustrated forms in the various species of the 

 Mesonacidae with advanced genal spines has them in the position they 

 occupy in this species. This species is represented in the collections 

 by about 15 specimens in all of which the spines are in the same 

 position, even though the cephala vary from 2.5 cm. to more than 

 4 cm. in width, indicating again that within these limits neither size 

 nor age of individuals causes variation in the position of the genal 

 spines. 



This species differs from M. fremonti first of all in the more ad- 

 vanced position of the long, slightly curved genal spines, which gives 



