NO. 2 CAMBRIAN FOSSILS FROM MOHAVE DESERT RESSER 3 



rather improbable that a strictly Arctic sea, narrow as it must have 

 been and on a continental mass of such low relief as then prevailed, 

 should approach so near the Pacific basin and not be connected with 

 it. The ^liddle Cambrian fauna of Bristol IMountain apparently 

 represents that usually characterized by the trilobite Dolichomeiopus 

 productiis, a fauna that is exceedingly widespread, extending from the 

 southern Appalachians to Greenland and from British Columbia to 

 Arizona, usually occupying a position somewhat below the middle of 

 the Middle Cambrian. This fauna again is commonly regarded as 

 Arctic rather than Pacific in origin. And so, while additional light is 

 shed on paleogeography by the Bristol Mountain fossils, we cannot 

 yet outline the exact boundaries of those early seas. 



DISCUSSION OF SOME AIESONACID GENERA 



I do not propose at this time to undertake the much needed revision 

 of the ]\Iesonacidae, but shall simply deal with questions raised by 

 the particular species under discussion. 



MESONACIS Walcott 1885 



Mcsonacis Walcott, 1885, Amer. Journ. Sci., 3d ser., Vol. 29, p. 328. (De- 

 scribed as a new genus.) 



Mesonacis Walcott, 1910. Smithsonian Misc. Coll., Vol. 53. No. 6, p. 261. 

 (General treatise on the entire family.) 



The distinctness of the genera Mesonacis and OleneUus has been 

 questioned, because many species previously referred to OleneUus 

 proved upon the discovery of additional specimens to have the extra 

 segments posterior to the fifteenth, and hence were transferred to 

 Mesonacis. Some of these transfers appear to be ill-founded since 

 no account was taken of the other generic features. 



At the present stage of the study I would suggest that both 

 Mesonacis and OleneUus are good genera, although there is consid- 

 erable difficulty in distinguishing the cephala or even the cephalon and 

 the first fifteen segments. Several dififerences may however be pointed 

 out. In Mesonacis the eyes are shorter and do not reach the occipital 

 ring ; also the rim around the head is narrower, particularly near the 

 genal angles. The main distinction, according to my present view, 

 is to be found in the character of the so-called rudimentary segments 

 that occur posterior to the fifteenth, with its large spine. Unfortu- 

 nately these most important anatomical features are infrequently 

 preserved, even though the percentage of entire shields to cephala is 

 considerably greater than in other trilobite groups. In Mesonacis all 



