BRACHIOFODA. 
2G7 
In the Devonian of North America we find species externally similar to 
Terebratula Guerangeri, and shall presently recur to them in considering the 
precise variation of structure which such forms exemplify. 
Further, there is evidence of the existence, in the Upper Helderberg 
fauna, of a small, smooth biconvex terebratuliform shell, with sloping 
cardinal margins which bears the same form of brachidium as Centronella 
glans-fagea* 
This type of brachial apparatus, however different may be the exterior of 
the shells bearing it, whether ribbed, striated or smooth, biconvex or plano¬ 
convex, has a definite time-value, and our present knowledge indicates that, 
beginning its existence in the Lower Helderberg, it is continued into the Oris- 
kany sandstone. Upper Helderberg and Hamilton groups without material 
modification. 
From this time onward the structure yielded to modifying influences. In 
the Centronella Julia of A. Winchell, a smooth, biconvex species from the Mar¬ 
shall group of Michigan, at the base of the Lower Carboniferous series, the 
anterior triangular plate of the brachidium bears a high vertical lamella extend¬ 
ing in both directions beyond the limits of that plate into the cavity of the 
pedicle-valve. 
Such forms must be kept apart from the earlier and typical Centronellas, as 
indicative of a progressed stage of development. The strongly plicated shells 
from the Productus limestone (Carboniferous) of the Salt-Range of India, which 
Waagen has described under the generic name Notothyris (though the struct¬ 
ure of their brachidia is not fully known), will undoubtedly prove to possess 
some modification of the Centronella type. Such a modification is evident in 
Derby’s Notothyris ? Smithi (loc. cit.) from the Devonian, a similarly plicated 
species which may prove to be an ancestral form of the Carboniferous 
Notothyris. 
Returning again to the genus Centronella and restricting our conception 
to forms similar to that regarded by its author as typical, the following diag¬ 
nosis may be deduced ; 
* Farther reference will be made to this shell. 
