GEOLOGICAL HISTORY 45 



remains of two Bivalves. One of these Bivalves 

 (Modioloides prtscus) 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 Strap avollina) of the 

 early Rhipidoglossa are found, as well as one 

 (Trochonema) 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 (Turbinidse) that belong to the more specialized 

 one-gilled section, while Holopcea and Scalites were 

 added to the Tsenioglossates. Pelecypods were still 

 rare, the Protobranchs were represented by Cteno- 

 donta, and the more specialized Filibranchs by Cyrto- 

 donta and Eopteria. The epoch also produced the 



