THE ORDOVICIAN PERIOD 



345 



subsequently expanded into considerable diversity, 

 tropods developed many 

 variations of the spiral 

 (Fig. 350) and flat-coiled 

 shells (Fig. 351), lacking 

 in fact only the orna- 

 mental spines and tu- 

 bercles of our modern 

 species. The two- 

 .. shelled mollusks. or pe- 



FIG. 348. A small 



horn coral (Strepte- lecypods, seem to have 



lasma) from the Or- ma d e glower progress J 

 do vician limestones. ., 



yet there are many of 

 Ordovician rocks (Fig. 



The gas- 



FIG. 349. Broken 

 fragment of one of 

 the earliest com- 

 pound corals, show- 

 ing several coales- 

 cent tubes each 

 built by an individ- 

 ual coral animal. 



them in certain 



352). Like the brachiopods, they first ap- 

 peared with simple unornamented shells, 

 gathering complexity of 

 structure and decora- 

 tion as they advanced. 

 The highest, and in some 

 respects the most re- 

 markable, group of mol- 

 lusks, the cephalopods, 

 makes its first appear- 

 ance in numbers in the 

 Ordovician strata. The 

 earliest types had 



FIG. 350.-AnOrdo- strai g ht tapering shells 



(Fig. 353), open at the 



larger end and divided 



by sagging partitions 



into a series of chambers. A seeming ad- FIG. 352. A small, 

 vance is shown in curved (Fig. 354) or even 

 tightly coiled shells (Fig. 355), which ap- 

 peared at this time. The remarkable folding of the dividing 

 partitions did not, however, set in until the Devonian. 



FIG. 351. A stout 

 flat-coiled gastro- 

 pod (Bellerophon) 

 common in the Or- 

 dovician period. 



vician gastropod 

 (Hormotoma) with 

 tall, spiral shell. 



plain pelecypod 

 (Ctenodonta). 



