nULLF.TlN UF THE NATUItAL HIHTOUV SOCIETY. 
gemnuda of the Protolenus fauna|. (see PL XV, figs. 1 a-d) in the 
sharp iimhonal ridge, and by the strong lateral ridges of the dorsal 
valv'e. From Acrofhyra proavia of the Etcheminian fauna (see PI. 
XIV, figs. 2 a-g) it is distinguished by the more central apex of 
the ventral valve, and by the prominent ridges of the interior of 
the dorsal valve, as well as by its larger size. From A. gemma, 
Bill. (Walcott)^ it is distinct by its smaller size, less proportionate 
height of the ventral valve, its narrow umbonal boss and its more 
obscure cardinal area, also by a difference in the internal markings 
of the dorsal valve. 
Acrotheta bisecta. pi. xvi. Figs. '2a to g. 
Occurs in the Upper Cambrain of Cape Breton (Dictyonema hori- 
zon) and IS described in a previous number of this Bulletin (vol. iv., 
p. 275). It agrees best with A. Baileyi, but differs from all the others 
by the strong median septum of the dorsal valve. 
Acrotreta. sp. 
A species of this genus occurs in the sandstone of E 2a at Young’s 
Point with TAnguJella Selwyni. It is rare, and only a dorsal valve 
has been found. 
Uevelopment of Acrotreta. 
This is one of the most conservative of the genera of the Cambrian 
and Ordovician. Though its species occur at intervals at various hori- 
zons in this system, the uniformity of size and sculpturing are remark- 
able. The latter consists of fine concentric striation, only visible with 
a strong lens. The size did not increase more than about four-fold in 
area in the vast space of time included in the Cambrian and Ordovi- 
an Systems. Contrast this with Paradoxides, which increased in area 
an hundred fold in the first two sub-faunas of the Paradoxides Zone. 
X Trans. N York Acad. Sci., xiv, p. 126, pi. v, fig. 5a to d. 
S> Nat, Hist. Soc. N. B. Bull, xviii, Vol. iv, p. 203, pi. iii, figs. 2a to /. 
U. S. Geol. 8urv. Bull. 30, p. 98, pi. viii, figs. 1,1a and b. 
