384 THE ORDOVICIAN PERIOD 



septa. The commonest shell of this type is Orthoceras, which is 

 a straight and very elongate cone (PL II, Fig. 12) and sometimes 

 attains a length of 10 feet ; the genus persists throughout the Palae- 

 ozoic and into the Mesozoic. Endoceras, which likewise has a 

 straight shell, with a curiously complex siphuncle, is confined to 

 the Ordovician. Besides these straight forms we find curved shells 

 like Cyrtoceras, shells like Lituites, coiled at one end, with a long, 

 straight terminal portion, resembling an Orthoceras with its smaller 

 end rolled up into a coil. Others again, like Trocholites, have the 

 shell coiled in a close, flat spiral. 



Vertebrata. — The curious, mail-clad Ostracoderms, primitive 

 vertebrates which somewhat resemble the fishes in appearance, 

 have been reported from Ordovician sandstones of Colorado. As 

 these remains are very imperfect and as the geological position of 

 the beds has been questioned, description of the Ostracoderms will 

 be deferred till a later chapter. Teeth of true fishes have been 

 found in the Ordovician of Europe. 



