374 THE CAMBRIAN PERIOD ^ 



order of Brachiopods, the Articulata, which have calcareous shells 

 connected by an elaborate hinge, became more common in the 

 Upper Cambrian. They soon grow vastly more numerous than 

 the Inarticulata, and throughout the post-Cambrian divisions of 

 the Palaeozoic their shells are found in incalculable numbers. 



The Mollusca are already represented by their principal divi- 

 sions. The Pelycypoda, or Bivalves, are of very small size and 

 found very scantily ; their variety and relative importance have 

 gone on increasing ever since Cambrian times. Gastropoda occur 

 in small numbers, especially in the Upper Cambrian. Fossils re- 

 ferred, with some doubt, to the Pteropoda are among the most 

 frequent of shells found in these rocks, but display no great 

 variety. The Cephalopoda, which are the highest group of mol- 

 luscs, are at present represented by two suborders ; in one, the 

 squids and cuttle-fishes (Dibrane/ii'ata), the shell is rudimentary 

 and internal ; while in the other ( Tetrabranchiata) the shell is 

 external. The latter kind of shell is divided by transverse septa 

 into chambers, which are connected by means of a tube, the 

 siphunde, the animal living only in the terminal chamber at 

 the mouth of the shell. The only living representative of this 

 group is the pearly Nautilus, but throughout Mesozoic and 

 Palaeozoic time there was a great variety of these chambered 

 shells. In the Cambrian the Cephalopods are very few and 

 almost confined to the uppermost part of the system. 



The Cambrian fauna displays steady progress, being distinctly 

 more advanced in the upper than in the lower division. The 

 Middle Cambrian fauna, so far as it is yet known, has not nearly 

 so many species as the Lower, but this is doubtless due to unfa- 

 vourable conditions of preservation. 



