250 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Mcsonacis occurs in association with OlcncUns, but I think that 

 Mesonacis-WkQ forms developed at an early epoch and that Mesonacis 

 vermontana is a survival of a stage in the evolution of Olenellus 

 that preceded ElUptocephala and Pccdeuinias. One of the conclusions 

 resulting from this study of the Mesonacidae is that we know only 

 a few of the representatives of the family, and of these only a very 

 few showing the younger stages of growth, and the entire dorsal 

 shield. 



MESONACID.^ AND PARADOXIN^ 



The family Mesonacidae is distinguished from the Paradoxina? 

 mainly by the presence in the latter of free cheeks separable on the 

 line of the facial sutures from the cranidium. In the Mesonacidae 

 the facial sutures are in a state of symphysis and the free cheeks 

 and cranidium are frequently not to be distinguished. 



STRATIGRAPHIC POSITION OF THE GENERA 

 AND SPECIES 



All of the known species of the Mesonacidae occur in the Lower 

 Cambrian or Georgian terrane. At the type locality in the town of 

 Georgia, Vermont, Olenellus and Mesonacis occur in the same 

 beds, and as far as known to me all known species of Olenellus, as 

 restricted, are from the upper zones of the Georgian terrane. 



In the accompanying table of genera and species the Lower Cam- 

 brian is arbitrarily divided into four divisions or zones, as follows : 



D — Olenellus, or upper zone. 



C = Callavia zone. 



B = Elliptocephala zone. 



A=Nevadia, or lower zone. 



In the Nevadia zone (A) we find the genus Nevadia [pi. 23] with 

 one species, also a species that is referred to Holniia, H. rozvei 

 [pi. 29]. 



In zone " B " which is above zone " A " ElUptocephala occurs, 

 also, doubtfully, IVaiineria and Olenellus. 



In zone " C" which is high up in the Lower Cambrian, but not 

 the upper zone, Callaz'ia is represented by five species, Mesonacis by 

 two, Holmia by two, Olenellus by one, and two are doubtfull\- re- 

 ferred to this horizon. 



In zone " D " Olenellus is represented by eleven species, Pce- 

 dcuniias by one, Jrainieria by two. Callavia by one, and Mesonacis 

 by one. 



