lot UULLKTIX OF THE NATURAL HISTORY SOCIETY. 
( Conthiued from pcujc .^01.) 
sible that the one referred to J. subsidua may be some other species, 
as it occurs with a different fauna than that of the original form, and 
apparently, should be much older. 
I have ventured to assign White’s A. subsidua to the Peltura Zone, 
Ixicause it appears to be the same with a species which occurs in the 
]\[t. Stephen Fauna ; this fauna contains an Ogygia and an Olenoides 
with other forms which appear to indicate this as the low’est horizon 
to which it should be assigned. White’s species is said by Director 
Walcott to occur with Asaphiscus and Olenoides, which also appear 
to be Upper Cambrian forms.^ 
Near the same horizon, or perhaps a little higher would come Bar- 
rande’s A, incohans which occurs in the “ Fauna of Hof ” equivalent 
to the Tremadoc Fauna. 
Among the Acrotheles there are several types of sculpturing of the 
surface of the valves. The most characteristic is that of fine, short, 
irregular wavy ridges, such as are found in A. Mattheivi and ^1. gramdata. 
Another type is represented in A. proles, A. gamagei and A. cf. coriacea 
wherein the ridges become more regularly concentric ; the valves in 
this group are larger, and the ventral less selliform than in the pre 
ceeding one. White’s description of A. subsidua would indicate that 
there is a third style of ornamentation in the latter species, in which 
the surface is papillose, yet with concentric lines of growth. 
From the time of its sudden appearance in the base of the Upper 
Etcheminian group, Acrothele continues to be common until we pass 
the liOwer Paradoxides beds; from this point upward they are rarely 
met with in Eastern Canada. It is thus more limited in range than 
Acrotreta which extends up into the Ordovician. Its range also dif- 
fers from that of Acrothyra, which is a common genus in the Lower 
Etcheminian, can be found even as far down as the Coldbrook, and 
also is present with Acrothele in the Upper Etcheminian, but hardly 
invades the Protolenus fauna ; where, as well as in the Lower Para- 
doxides beds, shells of Acrothele are common. 
Bull. U. S. (leol. Siirv. No. :30. p. 40. 
