ORDOVICIAN PERIOD 25 



Some very small animals, distantly related to lampshells POLYZOANS 

 and more highly connected, therefore, than the coral-related 

 graptolites were now in evidence. Whether any of them 

 lived in solitary state is unknown, but many colonies were 

 certainly in existence. Some of these extended themselves 

 over stones, shells, and other marine objects ; others had 

 taken root, and branched out in plant-like form, and resembled 

 the " seat-mats " and so-called " lace-coral " of our own 

 time. They certainly had a longer future before them than 

 Graptolites, for there is no doubt that the polyzoans of to-day 

 are their descendants, whilst Graptolite annals soon came to a 

 close. Polyzoans differed in character, for in some colonies 

 the zooids lived in round-mouthed and well-opened tubes 

 (Cyclostomata) ; whilst in other colonies probably an off- 

 shoot the tube orifice was more or less protected (Crypto- 

 stomata). There were, therefore, two developments competing 

 for Natural Selection ; and what the result would be, time 

 alone could show. 



Descendants of the Cambrian nautiloids in straight and CEPHALOPODS 

 slightly curved shells were numerous in Ordovician times. 

 In North American waters some forms attained enormous 

 dimensions, possessing shells a foot in diameter in front, and 

 twelve feet in length (Endoceras). Such giants, suggestive in 

 appearance of drifted tree-trunks, doubtless took terrible 

 toll of various forms of marine life. Owing to their ponderous 

 encasements they probably remained more or less stationary, 

 throwing out their tentacles when prey came within reach. 

 It is clear, however, that in this Period modifications were in 

 progress towards rendering the shells of nautiloids more 

 portable. Some forms were now appearing in shells partly 

 coiled in the uninhabited portions ; and the whole structure 

 thus assumed the outline of a pastoral staff (Lituites). The 

 shells of a few others were more or less completely coiled ; 

 and had, therefore, an outward resemblance to the shell of the 

 existing Nautilus (Trocholites). These modifications were 

 destined to revolutionise this class of life ; but as yet an 

 immense majority of the animals lived in shells of old fashion. 



Trilobites were making great progress ; and, during this TRILOBITES 

 Period, attained their climax. Some Cambrian genera had 



