THE ORDOVICIAN PERIOD 



343 



istic Ordovician brach- 

 iopod (Orthis). 



descended from those which lived there in the Cambrian 



period, and in part from others which 



had come in from elsewhere. 



Progress of the brachiopods and trilo- 



bites. Among the members of this 



fauna the brachiopods and trilobites still 



held a prominent position. The little Fig. 341. A character- 

 oval varieties of the former were at this 



time associated with larger types, many 



of which were ornamented with radiat- 

 ing ridges (Fig. 341). The species that 

 had hinged shells were more numerous 

 and even the spire-bearing group was 

 represented (Fig. 342). During the 

 Ordovician period the trilobites had 



FIG. 342. A common . . . '. 



Ordovician brachiopod risen rapidly to their culmination, and 

 with hookedbeak were even more numerous than in the 



(Rhynchotrema). . 



Cambrian. As we shall see, it was not 

 until the next period, however, that they ex- 

 hibited to the fullest their propensity for adopt- 

 ing queer forms and orna- 

 ments. Some of the Or- 

 dovician trilobites went 

 to the extreme of sim- 

 plicity (Fig. 343) in their 

 adornment; a few are 

 quite smooth, and are all 

 but devoid of even the 

 pair of furrows (Fig. 344) 

 which impart to most 

 members of the group 

 their trilobate aspect. 



FIG. 344. A relatively - T , T 



smooth trilobite (Iso- New ^oups of animals appear. In 

 telus) of the Ordovi- addition to the brachiopods and trilo- 



t D e^ t h C thS bites ' other 8^ rose to prominence 

 Cambrian types. in the Ordovician. Some were repre- 



FIQ. 343. A 

 remarkably 

 smooth t ri 1 o - 

 bite (Bumas- 

 tus) from the 

 Ordovi cian 

 rocks. 



