CCELENTERATA 



379 



is an expansion of that of the Cambrian, though but little direct 

 connection between the two can yet be traced, and evidently there 

 were great migrations of marine animals from some region which 

 cannot yet be identified. Several groups of invertebrates attained 

 their culmination and began to decline in the Ordovician, becom- 

 ing much less important in subsequent periods. Thus the Grapto- 

 lites, the Cystidean order of Echinoderms, the Pteropods among 

 Molluscs, and the Trilobites were never so abundant and so varied 

 as during this period. 



Plants. — In America no plants above the grade of seaweeds 

 have been discovered, but in Europe a few of the higher Crypto- 

 gams are doubtfully reported. The flora of the Devonian, how- 

 ever, renders it highly probable that land plants were already well 

 advanced in the Ordovician, and their remains may be discovered 

 at any time. This must remain a matter of accident, for the 

 known Ordovician rocks are all marine, which is not a favourable 

 circumstance for the preservation of land plants. Such discov- 

 eries have, indeed, already been reported, but the evidence for 

 them is not satisfactory. 



Foraminifera and Radiolaria have been found in sufficient num- 

 bers to prove that they were abundant in the Ordovician seas. 



Spongida. — Sponges are much more numerous and varied than 

 in the Cambrian. Of course it is only those sponges with skeletons 

 of lime or flint which can be well preserved in the fossil state, and 

 of these the Ordovician has many (PI. II, Fig. i). The horny 

 sponges, of which the common bath sponge is a familiar example, 

 are necessarily much rarer and less satisfactory as fossils. 



Coelenterata. — The Graptolites are very numerous and varied, 

 wherever conditions are favourable to their preservation, as in 

 fine-grained rocks with smooth bedding planes. The Ordovician 

 is the time of their culmination and is especially characterized by 

 the double forms, with rows of cells on both sides of the stem 

 (see PL II, Figs. 2, 3, 4). So abundant are the Graptolites that 

 in many parts of the system they are almost the only fossils and 

 are employed to divide the substages into zones. The few and 

 doubtful Cambrian Corals are succeeded by a considerable number 



