GEOLOGICAL HISTORY 45 
remains of two Bivalves. One of these Bivalves 
(Modioloides priscus) is known only by the internal 
cast, 2 millimetres long, and appears to belong to 
the Protobranchia, or oldest order, while the other 
(Fordilla) may prove to be a Bivalve crustacean and 
not a mollusc at all. ; 
In the Upper Cambrian further examples (Murchi- 
sonia, Cyrtolites, Owenella and Straparollina) of the 
early Rhipidoglossa are found, as well as one 
(Tvochonema) supposed to belong to the higher one- 
gilled section of that sub-order. Another Gastropod 
(Subulites) of doubtful affinities, but almost certainly 
belonging to the Pectinibranchs, and having a sipho- 
nostomatous shell, shows that three out of the four 
principal divisions of the Streptoneura were already 
represented at this early stage. Already, too, seven 
species of Cephalopoda had made their appearance. 
They all belong to the more primitive Nautiloidea, 
and, with one exception, the straight-shelled section 
of that group. 
During the succeeding Ordovician epoch the 
Aspidobranch Gastropods predominated, their ranks 
reinforced by representatives (Cyclonema) of the Top- 
Shells (Turbinidz) that belong to the more specialized 
one-gilled section, while Holopea and Scalites were 
added to the Tzenioglossates. Pelecypods were still 
rare, the Protobranchs were represented by Citeno- 
donta, and the more specialized Filibranchs by Cyrto- 
donta and Eopteria. The epoch also produced the 
